Note on omission of schema definition

spec/Section 2 -- Language.md

@@ -1018,26 +1018,46 @@ SchemaDefinition : schema { OperationTypeDefinition+ }
 
 OperationTypeDefinition : OperationType : NamedType
 
-A GraphQL Type System includes exactly one Schema Definition, which defines
-the type to be used for each operation.
+A GraphQL Type System includes at most one Schema Definition, which defines
+the *root types* to be used for each operation.
 
-In this example, a GraphQL schema is defined with both query and mutation types:
+In this example, a GraphQL schema is defined with both query and mutation
+root types:
 
 ```graphql
 schema {
-  query: QueryRoot
-  mutation: MutationRoot
+  query: MyQueryRootType
+  mutation: MyMutationRootType
 }
 
-type QueryRoot {
+type MyQueryRootType {
   someField: String
 }
 
-type MutationRoot {
+type MyMutationRootType {
   setSomeField(to: String): String
 }
 ```
 
+**Default Root Types**
+
+While any type can be the *root type* for a GraphQL query or mutation operation,
+GraphQL type system definitions can omit the schema definition when the query
+and mutation root types are named `Query` and `Mutation`, respectively.
+
+Similarly, when serializing a GraphQL schema using the type system language, a
+schema definition should be omitted if only uses the default root type names.
+
+This example describes a valid complete GraphQL schema, despite not explicitly
+including a schema definition. The `Query` type is presumed to be the query
+root type of the schema.
+
+```graphql
+type Query {
+  someField: String
+}
+```
+
 
 ### Type Definition
 

spec/Section 5 -- Validation.md

@@ -36,6 +36,10 @@ For this section of this schema, we will assume the following type system
 in order to demonstrate examples:
 
 ```
+type Query {
+  dog: Dog
+}
+
 enum DogCommand { SIT, DOWN, HEEL }
 
 type Dog implements Pet {
@@ -76,10 +80,6 @@ type Cat implements Pet {
 union CatOrDog = Cat | Dog
 union DogOrHuman = Dog | Human
 union HumanOrAlien = Human | Alien
-
-type QueryRoot {
-  dog: Dog
-}
 ```
 
 
@@ -532,7 +532,7 @@ and unions without subfields are disallowed.
 Let's assume the following additions to the query root type of the schema:
 
 ```
-extend type QueryRoot {
+extend type Query {
   human: Human
   pet: Pet
   catOrDog: CatOrDog
@@ -617,7 +617,7 @@ type Arguments {
   booleanListArgField(booleanListArg: [Boolean]!): [Boolean]
 }
 
-extend type QueryRoot {
+extend type Query {
   arguments: Arguments
 }
 ```
@@ -1479,7 +1479,7 @@ For these examples, consider the following typesystem additions:
 ```
 input ComplexInput { name: String, owner: String }
 
-extend type QueryRoot {
+extend type Query {
   findDog(complex: ComplexInput): Dog
   booleanList(booleanListArg: [Boolean!]): Boolean
 }