At the top of most data files there's a bibliography of sources, with the source mnemonic used at the specific data items, e.g., PB = Pepys Ballads, 1987, E = Euing Collection of English Broadside Ballads. Song files have title index at the beginning, and just click on item. No index for ABCs in tune file T1.HTM yet. S1 and S2 items are tunes for songs in the song text files.