diff --git a/spec/Section 4 -- Composition.md b/spec/Section 4 -- Composition.md
index 35d3b72..2d90a0c 100644
--- a/spec/Section 4 -- Composition.md	
+++ b/spec/Section 4 -- Composition.md	
@@ -14,6 +14,69 @@ run in sequence to produce the composite execution schema.
 
 ### Pre Merge Validation
 
+#### Semantic Equivalence of Types
+
+**Error Code**
+
+`TYPE_KIND_NOT_MERGEABLE`
+
+**Formal Specification**
+
+- Let {typesByName} be an empty unordered map of sets.
+- For each named type {type} across all source schemas:
+  - Let {name} be the name of {type}.
+  - Let {set} be the set in {typesByName} for {name}; if no such set exists,
+    create it as an empty set.
+  - Add {type} to {set}.
+- For each {typesByName} as {name} and {types}:
+  - Each pair of types in {types} must have the same kind.
+
+**Explanatory Text**
+
+The GraphQL Composite Schemas specification considers types with the same name
+across source schemas as semantically equivalent and mergeable.
+
+For the schema composition process to be able to merge types with the
+same name, the types must have the same kind. In this example we have two types
+called `User` which are both object types and are mergeable.
+
+```graphql example
+type User {
+  id: ID!
+  name: String!
+  displayName: String!
+  birthdate: String!
+}
+
+type User {
+  id: ID!
+  name: String!
+  reviews: [Review!]
+}
+```
+
+However, if the second type were a scalar type, the types would not be
+mergeable as they have different kinds.
+
+```graphql counter-example
+type User {
+  id: ID!
+  name: String!
+  displayName: String!
+  birthdate: String!
+}
+
+scalar User
+```
+
+All types with the same name must have the same kind to be mergeable.
+
+```graphql example
+scalar User
+
+scalar User
+```
+
 ### Merge
 
 ### Post Merge Validation