diff --git a/core-principles/api-first.md b/core-principles/api-first.md
index 726c9b9..4e098f0 100644
--- a/core-principles/api-first.md
+++ b/core-principles/api-first.md
@@ -1,2 +1,2 @@
 # API First
-API development always starts with API design.
\ No newline at end of file
+Development of an API MUST always start with API design. API Design MUST be formalized in a form of an API Description.
\ No newline at end of file
diff --git a/core-principles/apiary.md b/core-principles/apiary.md
index 734d4c3..aec6fd1 100644
--- a/core-principles/apiary.md
+++ b/core-principles/apiary.md
@@ -1,2 +1,3 @@
 # Apiary
-Every API description MUST to be stored in [Apiary](https://apiary.io/) under the ADIDAS GROUP team.
+Every API description MUST to be stored in [Apiary](https://apiary.io/) under the ADIDAS GROUP team. Apiary MUST be the single source of truth to learn about existing APIs within the organization. 
+
diff --git a/core-principles/openapi-specification.md b/core-principles/openapi-specification.md
index 1bd2353..0cbd555 100644
--- a/core-principles/openapi-specification.md
+++ b/core-principles/openapi-specification.md
@@ -1,2 +1,2 @@
 # OpenAPI Specification
-Every API has to be described using the [OpenAPI Specification (formerly known as Swagger Specification) version 2.0](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md).
+Every API MUST be described using API Description format. The API Description format used MUST be the [OpenAPI Specification (formerly known as Swagger Specification) version 2.0](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md).