The Subject index is divided up into 26 sections which are themselves sub-divided into 26 sub-sections, being initial letter (A-Z), and second letter (a to z).

Clicking on a letter in the Initial Letter Selector aligns the selected section and Second Letter Selector at the top of the column. Clicking on an a-z second letter aligns the menu at the selected two-letter selection. Clicking on the large Capitalized letter (the largest 'F' in the above example) in the Second Letter Selector re-positions the selected First Letter Selector to the top.

Any entries clicked upon in the Subject Index Menu will place the selected page into the main column, just left of the Subject Index.

Using it is easier than describing it...