diff --git a/docs/STYLEGUIDE.md b/docs/STYLEGUIDE.md
index 50351efe5b..bb8f715ae4 100644
--- a/docs/STYLEGUIDE.md
+++ b/docs/STYLEGUIDE.md
@@ -350,6 +350,10 @@ enum DifficultyLevel GetCurrentDifficultyLevel(void)
 }
 ```
 
+### Data file format
+
+External data files should use JSON.
+
 ## Principles
 
 ### Minimally Invasive