diff --git a/README.md b/README.md
index 64416123..c9106b9c 100644
--- a/README.md
+++ b/README.md
@@ -32,6 +32,7 @@ Tile38 is an open source (MIT licensed), in-memory geolocation data store, spati
 - Full [command line interface](#cli).
 - Leader / follower [replication](#replication).
 - In-memory database that persists on disk.
+- All coordinates are in [WGS 84 Web Mercator / EPSG:3857](#coordinate-system)
 
 ## Components
 - `tile38-server ` - The server
@@ -77,6 +78,12 @@ $ ./tile38-cli
 > help
 ```
 
+## Coordinate System
+It's important to note that the coordinate system Tile38 uses is 
+[WGS 84 Web Mercator](https://en.wikipedia.org/wiki/Web_Mercator), also known 
+as EPSG:3857. All distance are in meters and all calcuations are done on a spherical surface, 
+not a plane.
+
 ## <a name="cli"></a>Playing with Tile38
 
 Basic operations: