diff --git a/spec/Section 3 -- Type System.md b/spec/Section 3 -- Type System.md
index 44760852c..948f225ba 100644
--- a/spec/Section 3 -- Type System.md	
+++ b/spec/Section 3 -- Type System.md	
@@ -1089,20 +1089,50 @@ In this example, a directive is added to a `User` type without adding fields:
 extend type User @addedDirective
 ```
 
+Object type extensions may extend existing fields.
+
+In this example, we deprecate the field `id` on type `Query` by adding a
+`@deprecated` directive:
+
+```graphql example
+type Query {
+  id: String
+}
+```
+
+```graphql example
+extend type Query {
+  id: String @deprecated(reason: "Use newId")
+}
+```
+
 **Type Validation**
 
 Object type extensions have the potential to be invalid if incorrectly defined.
 
 1. The named type must already be defined and must be an Object type.
-2. The fields of an Object type extension must have unique names; no two fields
-   may share the same name.
-3. Any fields of an Object type extension must not be already defined on the
-   previous Object type.
-4. Any non-repeatable directives provided must not already apply to the previous
-   Object type.
-5. Any interfaces provided must not be already implemented by the previous
+2. For each field of an Object type extension:
+   1. The field must have a unique name within that Object type extension; no
+      two fields may share the same name.
+   2. If a field with the same name exists on the previous Object type:
+      1. The field type must match the previous definition exactly.
+      2. The field description must match the previous definition exactly.
+      3. Any non-repeatable directives provided must not already apply to the
+         previous definition.
+      4. For each argument of the field:
+         1. If an argument with the same name exists on the previous field
+            definition:
+            1. The argument type must match the previous definition exactly.
+            2. If the argument has a default value, it must match the previous
+               definition exactly.
+            3. The argument description must match the previous definition
+               exactly.
+            4. Any non-repeatable directives provided must not already apply to
+               the previous definition.
+3. Any non-repeatable directives provided must not already apply to the previous Object type.
+4. Any interfaces provided must not be already implemented by the previous
    Object type.
-6. The resulting extended object type must be a super-set of all interfaces it
+5. The resulting extended object type must be a super-set of all interfaces it
    implements.
 
 ## Interfaces
@@ -1340,22 +1370,53 @@ fields:
 extend interface NamedEntity @addedDirective
 ```
 
+Interface type extensions may extend existing fields.
+
+In this example, we deprecate the field `id` on type `Query` by adding a
+`@deprecated` directive:
+
+```graphql example
+interface Node {
+  id: ID
+}
+```
+
+```graphql example
+extend interface Node {
+  id: ID @deprecated(reason: "Use globalId instead")
+}
+```
+
 **Type Validation**
 
 Interface type extensions have the potential to be invalid if incorrectly
 defined.
 
 1. The named type must already be defined and must be an Interface type.
-2. The fields of an Interface type extension must have unique names; no two
-   fields may share the same name.
-3. Any fields of an Interface type extension must not be already defined on the
-   previous Interface type.
-4. Any Object or Interface type which implemented the previous Interface type
+2. For each field of an Interface type extension:
+   1. The field must have a unique name within that Interface type extension; no
+      two fields may share the same name.
+   2. If a field with the same name exists on the previous Interface type:
+      1. The field type must match the previous definition exactly.
+      2. The field description must match the previous definition exactly.
+      3. Any non-repeatable directives provided must not already apply to the
+         previous definition.
+      4. For each argument of the field:
+         1. If an argument with the same name exists on the previous field
+            definition:
+            1. The argument type must match the previous definition exactly.
+            2. If the argument has a default value, it must match the previous
+               definition exactly.
+            3. The argument description must match the previous definition
+               exactly.
+            4. Any non-repeatable directives provided must not already apply to
+               the previous definition.
+3. Any Object or Interface type which implemented the previous Interface type
    must also be a super-set of the fields of the Interface type extension (which
    may be due to Object type extension).
-5. Any non-repeatable directives provided must not already apply to the previous
+4. Any non-repeatable directives provided must not already apply to the previous
    Interface type.
-6. The resulting extended Interface type must be a super-set of all Interfaces
+5. The resulting extended Interface type must be a super-set of all Interfaces
    it implements.
 
 ## Unions