diff --git a/Sources/IdentifiedCollections/IdentifiedArray/IdentifiedArray+IdentifiedCollection.swift b/Sources/IdentifiedCollections/IdentifiedArray/IdentifiedArray+IdentifiedCollection.swift
new file mode 100644
index 0000000..3a3fd22
--- /dev/null
+++ b/Sources/IdentifiedCollections/IdentifiedArray/IdentifiedArray+IdentifiedCollection.swift
@@ -0,0 +1,44 @@
+import OrderedCollections
+
+extension IdentifiedArray: _IdentifiedCollection {
+  /// A read-only collection view for the ids contained in this array, as an `OrderedSet`.
+  ///
+  /// - Complexity: O(1)
+  @inlinable
+  @inline(__always)
+  public var ids: OrderedSet<ID> { self._dictionary.keys }
+}
+
+extension IdentifiedArray: _MutableIdentifiedCollection {
+  /// Accesses the value associated with the given id for reading and writing.
+  ///
+  /// This *id-based* subscript returns the element identified by the given id if found in the
+  /// array, or `nil` if no element is found.
+  ///
+  /// When you assign an element for an id and that element already exists, the array overwrites the
+  /// existing value in place. If the array doesn't contain the element, it is appended to the
+  /// array.
+  ///
+  /// If you assign `nil` for a given id, the array removes the element identified by that id.
+  ///
+  /// - Parameter id: The id to find in the array.
+  /// - Returns: The element associated with `id` if found in the array; otherwise, `nil`.
+  /// - Complexity: Looking up values in the array through this subscript has an expected complexity
+  ///   of O(1) hashing/comparison operations on average, if `ID` implements high-quality hashing.
+  ///   Updating the array also has an amortized expected complexity of O(1) -- although individual
+  ///   updates may need to copy or resize the array's underlying storage.
+  /// - Postcondition: Element identity must remain constant over modification. Modifying an
+  ///   element's id will cause a crash.
+  @inlinable
+  @inline(__always)
+  public subscript(id id: ID) -> Element? {
+    _read { yield self._dictionary[id] }
+    _modify {
+      yield &self._dictionary[id]
+      precondition(
+        self._dictionary[id].map { self._id($0) == id } ?? true,
+        "Element identity must remain constant"
+      )
+    }
+  }
+}
diff --git a/Sources/IdentifiedCollections/IdentifiedArray/IdentifiedArray.swift b/Sources/IdentifiedCollections/IdentifiedArray/IdentifiedArray.swift
index 36b1b8e..14e3624 100644
--- a/Sources/IdentifiedCollections/IdentifiedArray/IdentifiedArray.swift
+++ b/Sources/IdentifiedCollections/IdentifiedArray/IdentifiedArray.swift
@@ -272,13 +272,6 @@ public struct IdentifiedArray<ID: Hashable, Element> {
   @usableFromInline
   var _dictionary: OrderedDictionary<ID, Element>
 
-  /// A read-only collection view for the ids contained in this array, as an `OrderedSet`.
-  ///
-  /// - Complexity: O(1)
-  @inlinable
-  @inline(__always)
-  public var ids: OrderedSet<ID> { self._dictionary.keys }
-
   /// A read-only collection view for the elements contained in this array, as an `Array`.
   ///
   /// - Complexity: O(1)
@@ -297,38 +290,6 @@ public struct IdentifiedArray<ID: Hashable, Element> {
     self._dictionary = _dictionary
   }
 
-  /// Accesses the value associated with the given id for reading and writing.
-  ///
-  /// This *id-based* subscript returns the element identified by the given id if found in the
-  /// array, or `nil` if no element is found.
-  ///
-  /// When you assign an element for an id and that element already exists, the array overwrites the
-  /// existing value in place. If the array doesn't contain the element, it is appended to the
-  /// array.
-  ///
-  /// If you assign `nil` for a given id, the array removes the element identified by that id.
-  ///
-  /// - Parameter id: The id to find in the array.
-  /// - Returns: The element associated with `id` if found in the array; otherwise, `nil`.
-  /// - Complexity: Looking up values in the array through this subscript has an expected complexity
-  ///   of O(1) hashing/comparison operations on average, if `ID` implements high-quality hashing.
-  ///   Updating the array also has an amortized expected complexity of O(1) -- although individual
-  ///   updates may need to copy or resize the array's underlying storage.
-  /// - Postcondition: Element identity must remain constant over modification. Modifying an
-  ///   element's id will cause a crash.
-  @inlinable
-  @inline(__always)
-  public subscript(id id: ID) -> Element? {
-    _read { yield self._dictionary[id] }
-    _modify {
-      yield &self._dictionary[id]
-      precondition(
-        self._dictionary[id].map { self._id($0) == id } ?? true,
-        "Element identity must remain constant"
-      )
-    }
-  }
-
   @inlinable
   public func contains(_ element: Element) -> Bool {
     self._dictionary[self._id(element)] != nil
diff --git a/Sources/IdentifiedCollections/IdentifiedCollection.swift b/Sources/IdentifiedCollections/IdentifiedCollection.swift
new file mode 100644
index 0000000..242c803
--- /dev/null
+++ b/Sources/IdentifiedCollections/IdentifiedCollection.swift
@@ -0,0 +1,23 @@
+/// A collection of elements that can be uniquely identified.
+public protocol _IdentifiedCollection<ID, Element>: Collection {
+  /// A type that uniquely identifies elements in the collection.
+  associatedtype ID: Hashable
+
+  /// A type that describes all of the ids in the collection.
+  associatedtype IDs: Collection<ID>
+
+  /// A collection of ids associated with elements in the collection.
+  ///
+  /// This collection must contain elements equal to `map(\.id)`.
+  var ids: IDs { get }
+
+  /// Accesses the value associated with the given id for reading.
+  subscript(id id: ID) -> Element? { get }
+}
+
+/// A mutable collection of elements that can be uniquely identified.
+public protocol _MutableIdentifiedCollection<ID, Element>: _IdentifiedCollection, MutableCollection
+{
+  /// Accesses the value associated with the given id for reading.
+  subscript(id id: ID) -> Element? { get set }
+}