diff --git a/spec/Section 4 -- Composition.md b/spec/Section 4 -- Composition.md
index 1ea1a52..4ce4176 100644
--- a/spec/Section 4 -- Composition.md	
+++ b/spec/Section 4 -- Composition.md	
@@ -28,11 +28,11 @@ ERROR
 
 - Let {typeNames} be the set of all output type names from all source schemas.
 - For each {typeName} in {typeNames}
-  - Let {types} be the set of all types with the name {typeName} from all source
+  - Let {types} be the set of all types with the {typeName} from all source
     schemas.
   - Let {fieldNames} be the set of all field names from all {types}.
   - For each {fieldName} in {fieldNames}
-    - Let {fields} be the set of all fields with the name {fieldName} from all
+    - Let {fields} be the set of all fields with the {fieldName} from all
       {types}.
     - {FieldsAreMergeable(fields)} must be true.
 
@@ -115,6 +115,103 @@ type User {
 scalar Tag
 ```
 
+#### Output Field Argument Types Mergeable
+
+**Error Code**
+
+OUTPUT_FIELD_ARGUMENT_TYPES_NOT_MERGEABLE
+
+**Severity**
+
+ERROR
+
+**Formal Specification**
+
+- Let {typeNames} be the set of all output type names from all source schemas.
+- For each {typeName} in {typeNames}
+  - Let {types} be the set of all types with the {typeName} from all source
+    schemas.
+  - Let {fieldNames} be the set of all field names from all {types}.
+  - For each {fieldName} in {fieldNames}
+    - Let {fields} be the set of all fields with the {fieldName} from all
+      {types}.
+    - For each {field} in {fields}
+      - Let {argumentNames} be the set of all argument names from all {fields}.
+      - For each {argumentName} in {argumentNames}
+        - Let {arguments} be the set of all arguments with the {argumentName}
+          from all {fields}.
+        - For each pair of {argumentA} and {argumentB} in {arguments}
+          - ArgumentsAreMergeable({argumentA}, {argumentB}) must be true.
+
+ArgumentsAreMergeable(argumentA, argumentB):
+
+- Let {typeA} be the type of {argumentA}
+- Let {typeB} be the type of {argumentB}
+- {SameInputTypeShape(typeA, typeB)} must be true.
+
+**Explanatory Text**
+
+Fields on mergeable objects or interfaces that have the same name are considered
+semantically equivalent and mergeable when they have mergeable argument types.
+
+```graphql example
+type User {
+  field(argument: String): String
+}
+
+type User {
+  field(argument: String): String
+}
+```
+
+Arguments that differ on nullability of an argument type are mergeable.
+
+```graphql example
+type User {
+  field(argument: String!): String
+}
+
+type User {
+  field(argument: String): String
+}
+```
+
+```graphql example
+type User {
+  field(argument: [String!]): String
+}
+
+type User {
+  field(argument: [String]!): String
+}
+
+type User {
+  field(argument: [String]): String
+}
+```
+
+Arguments are not mergeable if the named types are different in kind or name.
+
+```graphql counter-example
+type User {
+  field(argument: String!): String
+}
+
+type User {
+  field(argument: DateTime): String
+}
+```
+
+```graphql counter-example
+type User {
+  field(argument: [String]): String
+}
+
+type User {
+  field(argument: [DateTime]): String
+}
+```
+
 ### Merge
 
 ### Post Merge Validation
@@ -125,7 +222,9 @@ scalar Tag
 
 `EMPTY_MERGED_OBJECT_TYPE`
 
-**Severity** ERROR
+**Severity**
+
+ERROR
 
 **Formal Specification**