A letter of the alphabet placed at the bottom of the first page of each sheet of a work, to denote, alphabetically, the order of the sheets.
It is customary to commence with B on the first sheet of the body of the work, and to go regularly through the alphabet, with the exception of the letters J, V, and W, which are never used as signatures; and which had, in fact, no existence in the alphabet at the time of the invention of printing; J, expressing both I and J; U, both U and V; and UU, the double letter W. If the work extend to more sheets in number than there are letters in the alphabet, the succeeding sheets go on with a second alphabet, which commences with A, and both the letters are usually given, in this manner, A A, or Aa, and sometimes, to avoid the repetition, thus, 2 A; if a third alphabet be necessary it is always, at the present day, placed with the number before it, as 3 A. The printer's first alphabet consists of twenty-two letters, and the second and succeeding ones of twenty-three.
As a guide to the bookbinder there are other signatures used in a sheet, besides the first — in a sheet of octavo the first page has B, the third has B 2, the fifth has B 3, and the seventh has B 4: in a sheet of twelves they are carried to B 6; B 5 being the first page of the offcut; and however numerous the pages may be in a sheet with one signature, when they are all inserted, they are continued to the last odd page before the middle of the sheet, but never carried beyond the middle. In general they are all omitted except the two first, to show the first fold of the paper, and the first on the offcut. Small capitals are more frequently used for signatures than large capitals, as disfiguring the foot of the page in a slighter manner.
Sometimes figures are used instead of letters, but not often; the Gentleman's Magazine is an instance.
I have extended these tables of Signatures and folios to 18mo., which is a size that has been much in use of late years; and in the table of 18mo. I have given it as usually imposed, as three half sheets of 12mo. with three signatures; the first signature in each sheet is with a capital letter, the intermediate signatures are small capitals.