diff --git a/src/viewer.js b/src/viewer.js
index d96f81c0..8711bd01 100644
--- a/src/viewer.js
+++ b/src/viewer.js
@@ -1782,7 +1782,7 @@ $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype,
      * @param {Element|String|Object} element - A reference to an element or an id for
      *      the element which will be overlayed. Or an Object specifying the configuration for the overlay
      * @param {OpenSeadragon.Point|OpenSeadragon.Rect} location - The point or
-     *      rectangle which will be overlayed.
+     *      rectangle which will be overlayed. This is a viewport relative location.
      * @param {OpenSeadragon.OverlayPlacement} placement - The position of the
      *      viewport which the location coordinates will be treated as relative
      *      to.
@@ -1843,7 +1843,7 @@ $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype,
      * @param {Element|String} element - A reference to an element or an id for
      *      the element which is overlayed.
      * @param {OpenSeadragon.Point|OpenSeadragon.Rect} location - The point or
-     *      rectangle which will be overlayed.
+     *      rectangle which will be overlayed. This is a viewport relative location.
      * @param {OpenSeadragon.OverlayPlacement} placement - The position of the
      *      viewport which the location coordinates will be treated as relative
      *      to.