diff --git a/src/Ouroboros-Mini_Protocols-Ping_Pong.thy b/src/Ouroboros-Mini_Protocols-Ping_Pong.thy
index 01cc38d..e396d52 100644
--- a/src/Ouroboros-Mini_Protocols-Ping_Pong.thy
+++ b/src/Ouroboros-Mini_Protocols-Ping_Pong.thy
@@ -19,7 +19,7 @@ datatype party =
   Client |
   Server
 
-subsection \<open>State Machines\<close>
+subsection \<open>State Machine\<close>
 
 datatype state =
   Idle |
diff --git a/src/Ouroboros-Mini_Protocols-Request_Response.thy b/src/Ouroboros-Mini_Protocols-Request_Response.thy
index 3e21e6b..6a442c8 100644
--- a/src/Ouroboros-Mini_Protocols-Request_Response.thy
+++ b/src/Ouroboros-Mini_Protocols-Request_Response.thy
@@ -23,7 +23,7 @@ datatype party =
   Client |
   Server
 
-subsection \<open>State Machines\<close>
+subsection \<open>State Machine\<close>
 
 datatype state =
   Idle |