diff --git a/README.md b/README.md
index 860f393..5e8782c 100644
--- a/README.md
+++ b/README.md
@@ -69,7 +69,7 @@ Options
  * -d _detail_: Detail at base zoom level (default 26-basezoom, ~0.5m, for tile resolution of 4096 if -z14)
  * -D _detail_: Detail at lower zoom levels (default 10, for tile resolution of 1024)
  * -m _detail_: Minimum detail that it will try if tiles are too big at regular detail (default 7)
- * -b _pixels_: Buffer size where features are duplicated from adjacent tiles (default 5)
+ * -b _pixels_: Buffer size where features are duplicated from adjacent tiles. Units are "screen pixels"--1/256th of the tile width or height. (default 5)
 
 ### Properties