diff --git a/experimental/class-features.md b/experimental/class-features.md
new file mode 100644
index 0000000..63c37e4
--- /dev/null
+++ b/experimental/class-features.md
@@ -0,0 +1,61 @@
+
+# Class Features
+
+This experimental extension covers three class features proposals: 
+[Class Fields], [Static Class Features] and [Private Methods].
+
+## ClassBody
+
+```js
+extend interface ClassBody {
+    body: [ MethodDefinition | PropertyDefinition ];
+}
+```
+
+## PropertyDefinition
+
+```js
+interface PropertyDefinition <: Node {
+    type: "PropertyDefinition";
+    key: Expression | PrivateIdentifier;
+    value: Expression | null;
+    computed: boolean;
+    static: boolean;
+}
+```
+
+- When `key` is a `PrivateIdentifier`, `computed` must be `false`.
+
+## MethodDefinition
+
+```js
+extend interface MethodDefinition {
+    key: Expression | PrivateIdentifier;
+}
+```
+
+- When `key` is a `PrivateIdentifier`, `computed` must be `false` and `kind` can not be `"constructor"`.
+
+### PrivateIdentifier
+
+```js
+interface PrivateIdentifier <: Node {
+    type: "PrivateIdentifier";
+    name: string;
+}
+```
+
+A private identifier refers to private class elements. For a private name `#a`, its `name` is `a`.
+
+```js
+extend interface MemberExpression {
+    property: Expression | PrivateIdentifier;
+}
+```
+
+- When `property` is a `PrivateIdentifier`, `computed` must be `false`.
+- When `object` is a `Super`, `property` can not be a `PrivateIdentifier`.
+
+[Class Fields]: https://github.com/tc39/proposal-class-fields
+[Static Class Features]: https://github.com/tc39/proposal-static-class-features/
+[Private Methods]: https://github.com/tc39/proposal-private-methods
diff --git a/experimental/private-fields-in-in.md b/experimental/private-fields-in-in.md
index 9a680ed..f6e9a1b 100644
--- a/experimental/private-fields-in-in.md
+++ b/experimental/private-fields-in-in.md
@@ -6,10 +6,10 @@
 
 ```js
 extend interface BinaryExpression <: Expression {
-    left: Expression | PrivateName;
+    left: Expression | PrivateIdentifier;
 }
 ```
 
-- `left` can be a private name (e.g. `#foo`) when `operator` is `"in"`.
+- `left` can be a private identifier (e.g. `#foo`) when `operator` is `"in"`.
 
 [proposal-private-fields-in-in]: https://github.com/tc39/proposal-private-fields-in-in