diff --git a/CMakeLists.txt b/CMakeLists.txt
index ac4e992..cad29a9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -40,6 +40,7 @@
   include/pybind11/detail/common.h
   include/pybind11/detail/descr.h
   include/pybind11/detail/init.h
+  include/pybind11/detail/internals.h
   include/pybind11/detail/typeid.h
   include/pybind11/attr.h
   include/pybind11/buffer_info.h
diff --git a/include/pybind11/cast.h b/include/pybind11/cast.h
index 805fea6..4c3abc1 100644
--- a/include/pybind11/cast.h
+++ b/include/pybind11/cast.h
@@ -13,6 +13,7 @@
 #include "pytypes.h"
 #include "detail/typeid.h"
 #include "detail/descr.h"
+#include "detail/internals.h"
 #include <array>
 #include <limits>
 #include <tuple>
@@ -32,108 +33,6 @@
 
 NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 NAMESPACE_BEGIN(detail)
-// Forward declarations:
-inline PyTypeObject *make_static_property_type();
-inline PyTypeObject *make_default_metaclass();
-inline PyObject *make_object_base_type(PyTypeObject *metaclass);
-struct value_and_holder;
-
-/// Additional type information which does not fit into the PyTypeObject
-struct type_info {
-    PyTypeObject *type;
-    const std::type_info *cpptype;
-    size_t type_size, holder_size_in_ptrs;
-    void *(*operator_new)(size_t);
-    void (*init_instance)(instance *, const void *);
-    void (*dealloc)(value_and_holder &v_h);
-    std::vector<PyObject *(*)(PyObject *, PyTypeObject *)> implicit_conversions;
-    std::vector<std::pair<const std::type_info *, void *(*)(void *)>> implicit_casts;
-    std::vector<bool (*)(PyObject *, void *&)> *direct_conversions;
-    buffer_info *(*get_buffer)(PyObject *, void *) = nullptr;
-    void *get_buffer_data = nullptr;
-    void *(*module_local_load)(PyObject *, const type_info *) = nullptr;
-    /* A simple type never occurs as a (direct or indirect) parent
-     * of a class that makes use of multiple inheritance */
-    bool simple_type : 1;
-    /* True if there is no multiple inheritance in this type's inheritance tree */
-    bool simple_ancestors : 1;
-    /* for base vs derived holder_type checks */
-    bool default_holder : 1;
-    /* true if this is a type registered with py::module_local */
-    bool module_local : 1;
-};
-
-PYBIND11_NOINLINE inline internals *&get_internals_ptr() {
-    static internals *internals_ptr = nullptr;
-    return internals_ptr;
-}
-
-PYBIND11_NOINLINE inline internals &get_internals() {
-    internals *&internals_ptr = get_internals_ptr();
-    if (internals_ptr)
-        return *internals_ptr;
-    handle builtins(PyEval_GetBuiltins());
-    const char *id = PYBIND11_INTERNALS_ID;
-    if (builtins.contains(id) && isinstance<capsule>(builtins[id])) {
-        internals_ptr = *static_cast<internals **>(capsule(builtins[id]));
-
-        // We loaded builtins through python's builtins, which means that our error_already_set and
-        // builtin_exception may be different local classes than the ones set up in the initial
-        // exception translator, below, so add another for our local exception classes.
-        //
-        // stdlibc++ doesn't require this (types there are identified only by name)
-        #if !defined(__GLIBCXX__)
-        internals_ptr->registered_exception_translators.push_front(
-            [](std::exception_ptr p) -> void {
-                try {
-                    if (p) std::rethrow_exception(p);
-                } catch (error_already_set &e)       { e.restore();   return;
-                } catch (const builtin_exception &e) { e.set_error(); return;
-                }
-            }
-        );
-        #endif
-    } else {
-        internals_ptr = new internals();
-        #if defined(WITH_THREAD)
-            PyEval_InitThreads();
-            PyThreadState *tstate = PyThreadState_Get();
-            internals_ptr->tstate = PyThread_create_key();
-            PyThread_set_key_value(internals_ptr->tstate, tstate);
-            internals_ptr->istate = tstate->interp;
-        #endif
-        builtins[id] = capsule(&internals_ptr);
-        internals_ptr->registered_exception_translators.push_front(
-            [](std::exception_ptr p) -> void {
-                try {
-                    if (p) std::rethrow_exception(p);
-                } catch (error_already_set &e)           { e.restore();                                    return;
-                } catch (const builtin_exception &e)     { e.set_error();                                  return;
-                } catch (const std::bad_alloc &e)        { PyErr_SetString(PyExc_MemoryError,   e.what()); return;
-                } catch (const std::domain_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return;
-                } catch (const std::invalid_argument &e) { PyErr_SetString(PyExc_ValueError,    e.what()); return;
-                } catch (const std::length_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return;
-                } catch (const std::out_of_range &e)     { PyErr_SetString(PyExc_IndexError,    e.what()); return;
-                } catch (const std::range_error &e)      { PyErr_SetString(PyExc_ValueError,    e.what()); return;
-                } catch (const std::exception &e)        { PyErr_SetString(PyExc_RuntimeError,  e.what()); return;
-                } catch (...) {
-                    PyErr_SetString(PyExc_RuntimeError, "Caught an unknown exception!");
-                    return;
-                }
-            }
-        );
-        internals_ptr->static_property_type = make_static_property_type();
-        internals_ptr->default_metaclass = make_default_metaclass();
-        internals_ptr->instance_base = make_object_base_type(internals_ptr->default_metaclass);
-    }
-    return *internals_ptr;
-}
-
-// Works like internals.registered_types_cpp, but for module-local registered types:
-PYBIND11_NOINLINE inline type_map<void *> &registered_local_types_cpp() {
-    static type_map<void *> locals{};
-    return locals;
-}
 
 /// A life support system for temporary objects created by `type_caster::load()`.
 /// Adding a patient will keep it alive up until the enclosing function returns.
diff --git a/include/pybind11/detail/common.h b/include/pybind11/detail/common.h
index 6c914db..feefc12 100644
--- a/include/pybind11/detail/common.h
+++ b/include/pybind11/detail/common.h
@@ -205,8 +205,6 @@
 #define PYBIND11_TRY_NEXT_OVERLOAD ((PyObject *) 1) // special failure return code
 #define PYBIND11_STRINGIFY(x) #x
 #define PYBIND11_TOSTRING(x) PYBIND11_STRINGIFY(x)
-#define PYBIND11_INTERNALS_ID "__pybind11_" \
-    PYBIND11_TOSTRING(PYBIND11_VERSION_MAJOR) "_" PYBIND11_TOSTRING(PYBIND11_VERSION_MINOR) "__"
 
 /** \rst
     ***Deprecated in favor of PYBIND11_MODULE***
@@ -442,73 +440,6 @@
 
 static_assert(std::is_standard_layout<instance>::value, "Internal error: `pybind11::detail::instance` is not standard layout!");
 
-struct overload_hash {
-    inline size_t operator()(const std::pair<const PyObject *, const char *>& v) const {
-        size_t value = std::hash<const void *>()(v.first);
-        value ^= std::hash<const void *>()(v.second)  + 0x9e3779b9 + (value<<6) + (value>>2);
-        return value;
-    }
-};
-
-// Python loads modules by default with dlopen with the RTLD_LOCAL flag; under libc++ and possibly
-// other stls, this means `typeid(A)` from one module won't equal `typeid(A)` from another module
-// even when `A` is the same, non-hidden-visibility type (e.g. from a common include).  Under
-// stdlibc++, this doesn't happen: equality and the type_index hash are based on the type name,
-// which works.  If not under a known-good stl, provide our own name-based hasher and equality
-// functions that use the type name.
-#if defined(__GLIBCXX__)
-inline bool same_type(const std::type_info &lhs, const std::type_info &rhs) { return lhs == rhs; }
-using type_hash = std::hash<std::type_index>;
-using type_equal_to = std::equal_to<std::type_index>;
-#else
-inline bool same_type(const std::type_info &lhs, const std::type_info &rhs) {
-    return lhs.name() == rhs.name() ||
-        std::strcmp(lhs.name(), rhs.name()) == 0;
-}
-struct type_hash {
-    size_t operator()(const std::type_index &t) const {
-        size_t hash = 5381;
-        const char *ptr = t.name();
-        while (auto c = static_cast<unsigned char>(*ptr++))
-            hash = (hash * 33) ^ c;
-        return hash;
-    }
-};
-struct type_equal_to {
-    bool operator()(const std::type_index &lhs, const std::type_index &rhs) const {
-        return lhs.name() == rhs.name() ||
-            std::strcmp(lhs.name(), rhs.name()) == 0;
-    }
-};
-#endif
-
-template <typename value_type>
-using type_map = std::unordered_map<std::type_index, value_type, type_hash, type_equal_to>;
-
-/// Internal data structure used to track registered instances and types
-struct internals {
-    type_map<void *> registered_types_cpp; // std::type_index -> type_info
-    std::unordered_map<PyTypeObject *, std::vector<type_info *>> registered_types_py; // PyTypeObject* -> base type_info(s)
-    std::unordered_multimap<const void *, instance*> registered_instances; // void * -> instance*
-    std::unordered_set<std::pair<const PyObject *, const char *>, overload_hash> inactive_overload_cache;
-    type_map<std::vector<bool (*)(PyObject *, void *&)>> direct_conversions;
-    std::unordered_map<const PyObject *, std::vector<PyObject *>> patients;
-    std::forward_list<void (*) (std::exception_ptr)> registered_exception_translators;
-    std::unordered_map<std::string, void *> shared_data; // Custom data to be shared across extensions
-    std::vector<PyObject *> loader_patient_stack; // Used by `loader_life_support`
-    std::forward_list<std::string> static_strings; // Stores the std::strings backing detail::c_str()
-    PyTypeObject *static_property_type;
-    PyTypeObject *default_metaclass;
-    PyObject *instance_base;
-#if defined(WITH_THREAD)
-    decltype(PyThread_create_key()) tstate = 0; // Usually an int but a long on Cygwin64 with Python 3.x
-    PyInterpreterState *istate = nullptr;
-#endif
-};
-
-/// Return a reference to the current 'internals' information
-inline internals &get_internals();
-
 /// from __cpp_future__ import (convenient aliases from C++14/17)
 #if defined(PYBIND11_CPP14) && (!defined(_MSC_VER) || _MSC_VER >= 1910)
 using std::enable_if_t;
@@ -716,47 +647,8 @@
 #define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) pybind11::detail::expand_side_effects{ ((PATTERN), void(), false)..., false }
 #endif
 
-/// Constructs a std::string with the given arguments, stores it in `internals`, and returns its
-/// `c_str()`.  Such strings objects have a long storage duration -- the internal strings are only
-/// cleared when the program exits or after interpreter shutdown (when embedding), and so are
-/// suitable for c-style strings needed by Python internals (such as PyTypeObject's tp_name).
-template <typename... Args> const char *c_str(Args &&...args) {
-    auto &strings = get_internals().static_strings;
-    strings.emplace_front(std::forward<Args>(args)...);
-    return strings.front().c_str();
-}
-
 NAMESPACE_END(detail)
 
-/// Returns a named pointer that is shared among all extension modules (using the same
-/// pybind11 version) running in the current interpreter. Names starting with underscores
-/// are reserved for internal usage. Returns `nullptr` if no matching entry was found.
-inline PYBIND11_NOINLINE void* get_shared_data(const std::string& name) {
-    auto& internals = detail::get_internals();
-    auto it = internals.shared_data.find(name);
-    return it != internals.shared_data.end() ? it->second : nullptr;
-}
-
-/// Set the shared data that can be later recovered by `get_shared_data()`.
-inline PYBIND11_NOINLINE void *set_shared_data(const std::string& name, void *data) {
-    detail::get_internals().shared_data[name] = data;
-    return data;
-}
-
-/// Returns a typed reference to a shared data entry (by using `get_shared_data()`) if
-/// such entry exists. Otherwise, a new object of default-constructible type `T` is
-/// added to the shared data under the given name and a reference to it is returned.
-template<typename T> T& get_or_create_shared_data(const std::string& name) {
-    auto& internals = detail::get_internals();
-    auto it = internals.shared_data.find(name);
-    T* ptr = (T*) (it != internals.shared_data.end() ? it->second : nullptr);
-    if (!ptr) {
-        ptr = new T();
-        internals.shared_data[name] = ptr;
-    }
-    return *ptr;
-}
-
 /// C++ bindings of builtin Python exceptions
 class builtin_exception : public std::runtime_error {
 public:
diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
new file mode 100644
index 0000000..73f34df
--- /dev/null
+++ b/include/pybind11/detail/internals.h
@@ -0,0 +1,232 @@
+/*
+    pybind11/detail/internals.h: Internal data structure and related functions
+
+    Copyright (c) 2017 Wenzel Jakob <wenzel.jakob@epfl.ch>
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#pragma once
+
+#include "../pytypes.h"
+
+NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
+NAMESPACE_BEGIN(detail)
+// Forward declarations
+inline PyTypeObject *make_static_property_type();
+inline PyTypeObject *make_default_metaclass();
+inline PyObject *make_object_base_type(PyTypeObject *metaclass);
+
+// Python loads modules by default with dlopen with the RTLD_LOCAL flag; under libc++ and possibly
+// other STLs, this means `typeid(A)` from one module won't equal `typeid(A)` from another module
+// even when `A` is the same, non-hidden-visibility type (e.g. from a common include).  Under
+// libstdc++, this doesn't happen: equality and the type_index hash are based on the type name,
+// which works.  If not under a known-good stl, provide our own name-based hash and equality
+// functions that use the type name.
+#if defined(__GLIBCXX__)
+inline bool same_type(const std::type_info &lhs, const std::type_info &rhs) { return lhs == rhs; }
+using type_hash = std::hash<std::type_index>;
+using type_equal_to = std::equal_to<std::type_index>;
+#else
+inline bool same_type(const std::type_info &lhs, const std::type_info &rhs) {
+    return lhs.name() == rhs.name() || std::strcmp(lhs.name(), rhs.name()) == 0;
+}
+
+struct type_hash {
+    size_t operator()(const std::type_index &t) const {
+        size_t hash = 5381;
+        const char *ptr = t.name();
+        while (auto c = static_cast<unsigned char>(*ptr++))
+            hash = (hash * 33) ^ c;
+        return hash;
+    }
+};
+
+struct type_equal_to {
+    bool operator()(const std::type_index &lhs, const std::type_index &rhs) const {
+        return lhs.name() == rhs.name() || std::strcmp(lhs.name(), rhs.name()) == 0;
+    }
+};
+#endif
+
+template <typename value_type>
+using type_map = std::unordered_map<std::type_index, value_type, type_hash, type_equal_to>;
+
+struct overload_hash {
+    inline size_t operator()(const std::pair<const PyObject *, const char *>& v) const {
+        size_t value = std::hash<const void *>()(v.first);
+        value ^= std::hash<const void *>()(v.second)  + 0x9e3779b9 + (value<<6) + (value>>2);
+        return value;
+    }
+};
+
+/// Internal data structure used to track registered instances and types.
+struct internals {
+    type_map<void *> registered_types_cpp; // std::type_index -> type_info
+    std::unordered_map<PyTypeObject *, std::vector<type_info *>> registered_types_py; // PyTypeObject* -> base type_info(s)
+    std::unordered_multimap<const void *, instance*> registered_instances; // void * -> instance*
+    std::unordered_set<std::pair<const PyObject *, const char *>, overload_hash> inactive_overload_cache;
+    type_map<std::vector<bool (*)(PyObject *, void *&)>> direct_conversions;
+    std::unordered_map<const PyObject *, std::vector<PyObject *>> patients;
+    std::forward_list<void (*) (std::exception_ptr)> registered_exception_translators;
+    std::unordered_map<std::string, void *> shared_data; // Custom data to be shared across extensions
+    std::vector<PyObject *> loader_patient_stack; // Used by `loader_life_support`
+    std::forward_list<std::string> static_strings; // Stores the std::strings backing detail::c_str()
+    PyTypeObject *static_property_type;
+    PyTypeObject *default_metaclass;
+    PyObject *instance_base;
+#if defined(WITH_THREAD)
+    decltype(PyThread_create_key()) tstate = 0; // Usually an int but a long on Cygwin64 with Python 3.x
+    PyInterpreterState *istate = nullptr;
+#endif
+};
+
+/// Additional type information which does not fit into the PyTypeObject.
+struct type_info {
+    PyTypeObject *type;
+    const std::type_info *cpptype;
+    size_t type_size, holder_size_in_ptrs;
+    void *(*operator_new)(size_t);
+    void (*init_instance)(instance *, const void *);
+    void (*dealloc)(value_and_holder &v_h);
+    std::vector<PyObject *(*)(PyObject *, PyTypeObject *)> implicit_conversions;
+    std::vector<std::pair<const std::type_info *, void *(*)(void *)>> implicit_casts;
+    std::vector<bool (*)(PyObject *, void *&)> *direct_conversions;
+    buffer_info *(*get_buffer)(PyObject *, void *) = nullptr;
+    void *get_buffer_data = nullptr;
+    void *(*module_local_load)(PyObject *, const type_info *) = nullptr;
+    /* A simple type never occurs as a (direct or indirect) parent
+     * of a class that makes use of multiple inheritance */
+    bool simple_type : 1;
+    /* True if there is no multiple inheritance in this type's inheritance tree */
+    bool simple_ancestors : 1;
+    /* for base vs derived holder_type checks */
+    bool default_holder : 1;
+    /* true if this is a type registered with py::module_local */
+    bool module_local : 1;
+};
+
+#define PYBIND11_INTERNALS_ID "__pybind11_" \
+    PYBIND11_TOSTRING(PYBIND11_VERSION_MAJOR) "_" PYBIND11_TOSTRING(PYBIND11_VERSION_MINOR) "__"
+
+/// Each module locally stores a pointer to the `internals` data. The data
+/// itself is shared among modules with the same `PYBIND11_INTERNALS_ID`.
+PYBIND11_NOINLINE inline internals *&get_internals_ptr() {
+    static internals *internals_ptr = nullptr;
+    return internals_ptr;
+}
+
+/// Return a reference to the current `internals` data
+PYBIND11_NOINLINE inline internals &get_internals() {
+    auto *&internals_ptr = get_internals_ptr();
+    if (internals_ptr)
+        return *internals_ptr;
+
+    constexpr auto *id = PYBIND11_INTERNALS_ID;
+    auto builtins = handle(PyEval_GetBuiltins());
+    if (builtins.contains(id) && isinstance<capsule>(builtins[id])) {
+        internals_ptr = *static_cast<internals **>(capsule(builtins[id]));
+
+        // We loaded builtins through python's builtins, which means that our `error_already_set`
+        // and `builtin_exception` may be different local classes than the ones set up in the
+        // initial exception translator, below, so add another for our local exception classes.
+        //
+        // libstdc++ doesn't require this (types there are identified only by name)
+#if !defined(__GLIBCXX__)
+        internals_ptr->registered_exception_translators.push_front(
+            [](std::exception_ptr p) -> void {
+                try {
+                    if (p) std::rethrow_exception(p);
+                } catch (error_already_set &e)       { e.restore();   return;
+                } catch (const builtin_exception &e) { e.set_error(); return;
+                }
+            }
+        );
+#endif
+    } else {
+        internals_ptr = new internals();
+#if defined(WITH_THREAD)
+        PyEval_InitThreads();
+        PyThreadState *tstate = PyThreadState_Get();
+        internals_ptr->tstate = PyThread_create_key();
+        PyThread_set_key_value(internals_ptr->tstate, tstate);
+        internals_ptr->istate = tstate->interp;
+#endif
+        builtins[id] = capsule(&internals_ptr);
+        internals_ptr->registered_exception_translators.push_front(
+            [](std::exception_ptr p) -> void {
+                try {
+                    if (p) std::rethrow_exception(p);
+                } catch (error_already_set &e)           { e.restore();                                    return;
+                } catch (const builtin_exception &e)     { e.set_error();                                  return;
+                } catch (const std::bad_alloc &e)        { PyErr_SetString(PyExc_MemoryError,   e.what()); return;
+                } catch (const std::domain_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+                } catch (const std::invalid_argument &e) { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+                } catch (const std::length_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+                } catch (const std::out_of_range &e)     { PyErr_SetString(PyExc_IndexError,    e.what()); return;
+                } catch (const std::range_error &e)      { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+                } catch (const std::exception &e)        { PyErr_SetString(PyExc_RuntimeError,  e.what()); return;
+                } catch (...) {
+                    PyErr_SetString(PyExc_RuntimeError, "Caught an unknown exception!");
+                    return;
+                }
+            }
+        );
+        internals_ptr->static_property_type = make_static_property_type();
+        internals_ptr->default_metaclass = make_default_metaclass();
+        internals_ptr->instance_base = make_object_base_type(internals_ptr->default_metaclass);
+    }
+    return *internals_ptr;
+}
+
+/// Works like `internals.registered_types_cpp`, but for module-local registered types:
+PYBIND11_NOINLINE inline type_map<void *> &registered_local_types_cpp() {
+    static type_map<void *> locals{};
+    return locals;
+}
+
+/// Constructs a std::string with the given arguments, stores it in `internals`, and returns its
+/// `c_str()`.  Such strings objects have a long storage duration -- the internal strings are only
+/// cleared when the program exits or after interpreter shutdown (when embedding), and so are
+/// suitable for c-style strings needed by Python internals (such as PyTypeObject's tp_name).
+template <typename... Args>
+const char *c_str(Args &&...args) {
+    auto &strings = get_internals().static_strings;
+    strings.emplace_front(std::forward<Args>(args)...);
+    return strings.front().c_str();
+}
+
+NAMESPACE_END(detail)
+
+/// Returns a named pointer that is shared among all extension modules (using the same
+/// pybind11 version) running in the current interpreter. Names starting with underscores
+/// are reserved for internal usage. Returns `nullptr` if no matching entry was found.
+inline PYBIND11_NOINLINE void *get_shared_data(const std::string &name) {
+    auto &internals = detail::get_internals();
+    auto it = internals.shared_data.find(name);
+    return it != internals.shared_data.end() ? it->second : nullptr;
+}
+
+/// Set the shared data that can be later recovered by `get_shared_data()`.
+inline PYBIND11_NOINLINE void *set_shared_data(const std::string &name, void *data) {
+    detail::get_internals().shared_data[name] = data;
+    return data;
+}
+
+/// Returns a typed reference to a shared data entry (by using `get_shared_data()`) if
+/// such entry exists. Otherwise, a new object of default-constructible type `T` is
+/// added to the shared data under the given name and a reference to it is returned.
+template<typename T>
+T &get_or_create_shared_data(const std::string &name) {
+    auto &internals = detail::get_internals();
+    auto it = internals.shared_data.find(name);
+    T *ptr = (T *) (it != internals.shared_data.end() ? it->second : nullptr);
+    if (!ptr) {
+        ptr = new T();
+        internals.shared_data[name] = ptr;
+    }
+    return *ptr;
+}
+
+NAMESPACE_END(PYBIND11_NAMESPACE)
diff --git a/setup.py b/setup.py
index c67fa02..51ab47d 100644
--- a/setup.py
+++ b/setup.py
@@ -16,6 +16,7 @@
         'include/pybind11/detail/common.h',
         'include/pybind11/detail/descr.h',
         'include/pybind11/detail/init.h',
+        'include/pybind11/detail/internals.h',
         'include/pybind11/detail/typeid.h'
         'include/pybind11/attr.h',
         'include/pybind11/buffer_info.h',