For reasons which I trust will remain sufficiently obscure, I have been in need of verifying whether Tolkien had settled on a particular character name (e.g. Sauron, who went through quite a variety of forms and identities before settling down) or invented or adopted a particular term (e.g. Middle-earth, which he didn't start using until well into the writing of The Lord of the Rings) by a particular date.
The ideal source for this is the single-alphabetization compiled index to the 12 volumes of The History of Middle-earth, one of my most useful reference volumes; unfortunately for this purpose, Christopher Tolkien indexed his editorial references indistinguishably from the uses of the same terms in his father's texts. So seeing that a name appears in, say, vol. 4 doesn't mean JRRT had started using it yet; I have to look up the individual page references to tell for sure, though sometimes their sheer number or character provides a clue.
I am still without a computer. (See earlier entry.) Consequently you are not reading this; it does not exist.