diff --git a/ui/bookmarks.js b/ui/bookmarks.js
index df4f67d4..eb54a64b 100755
--- a/ui/bookmarks.js
+++ b/ui/bookmarks.js
@@ -6,7 +6,8 @@
 
 // list of bookmarked gids...
 //
-// NOTE: this must be sorted in the same order as DATA.order
+// NOTE: this is a sparse list, see marks.js MARKED for more info and
+// 		motivation...
 var BOOKMARKS = [] 
 
 // bookmark data
diff --git a/ui/marks.js b/ui/marks.js
index 41b645cf..389f208c 100755
--- a/ui/marks.js
+++ b/ui/marks.js
@@ -6,7 +6,21 @@
 
 //var DEBUG = DEBUG != null ? DEBUG : true
 
-// NOTE: this must be sorted in the same order as DATA.order
+// NOTE: this is a sparse list, i.e. all elements are in the same 
+// 		position as they are in DATA.order, and the unmarked elements
+// 		are undefined.
+// 		This is done because:
+// 			- it drasticly simplifies adding, removing and access as 
+// 				there is no searching and checking involved, just insert
+// 				to the same spot as in order and you are safe.
+// 			- trivial sorting
+// 			- less maintenance and sync
+// 		The tradeoff being:
+// 			- more memory usage
+// 			- load/save conversion to keep the json	data "packed".
+// NOTE: it would appear that JS is designed with sparse lists in mind:
+// 		- all iterators (map, filter, forEach, ..) skip undefined values
+// 		- really fast
 var MARKED = []
 
 var MARKED_FILE_DEFAULT = 'marked.json'
diff --git a/ui/tags.js b/ui/tags.js
index 5b08ff8c..ec2eff92 100755
--- a/ui/tags.js
+++ b/ui/tags.js
@@ -17,6 +17,8 @@ var UNSORTED_TAG = 'unsorted'
 // 		...
 // 	}
 //
+// NOTE: unlike MARKED and BOOKMARKS the gid lists here are not sparse.
+// XXX do we need to make this sparse??
 var TAGS = {}