diff --git a/save/region/mca.go b/save/region/mca.go
index e57b318..fa3295e 100644
--- a/save/region/mca.go
+++ b/save/region/mca.go
@@ -20,6 +20,7 @@ type Region struct {
 }
 
 // In calculate chunk's coordinates relative to region
+// 计算chunk在region中的相对坐标。即，除以32并取余。
 func In(cx, cz int) (int, int) {
 	// c & (32-1)
 	// is equal to: