diff --git a/src/base/TilesRendererBase.js b/src/base/TilesRendererBase.js
index be097268..463aa9fa 100644
--- a/src/base/TilesRendererBase.js
+++ b/src/base/TilesRendererBase.js
@@ -248,6 +248,8 @@ export class TilesRendererBase {
 
 	markTileUsed( tile ) {
 
+		// save the tile in a separate "used set" so we can mark it as unused
+		// before the next tile set traversal
 		this.usedSet.add( tile );
 		this.lruCache.add( tile );