Wikipedia's article on the Halifax Gibbet says, with several citations:
…ancient custom and law gave the Lord of the Manor the authority to execute summarily by decapitation any thief caught with stolen goods to the value of 13½d or more…
and quotes A.T. Carter History of English Legal Institutions, apparently quoting the original statute:
… cloth or any other commodity to the value of 13½d, that they shall after three market days or meeting days within the town of Halifax after such his apprehension, and being condemned he shall be taken to the gibbet and there have his head cut off from his body.
Even in pre-decimalization British currency, 13½ pence is not a round number; it is 1⅛ shillings. My question is: What is the origin of this oddity?
The only thing I can imagine is that the criterion was originally established at some round amount of goods, say something like 10 ells of cloth, and the commodity amount was later converted to a money amount at the prevailing commodity price.
(The 13½ is not Wikipedia vandalism; I found the same amount cited in William Smith, ed. Old Yorkshire (Longmans, Green & Co., London, 1887) p.137, and in an article, “The Halifax Gibbet-law” in The Saturday Magazine #132 (p. 32) of July 26, 1834.)
Jeffrey Kegler points out that the amount 13½d appears in English folklore in at least one other place; it is the traditional “hangman's wages”. For example we find in Samuel Pegge Curialia Miscellanea; Or, Anecdotes of Old Times (J. Nichols, London, 1818):
HANGMAN'S WAGES. The vulgar notion, though it will not appear to be a vulgar error, is, that Thirteen Pence Halfpenny is the fee of the Executioner in the common line of business at Tyburn, and therefore is called Hangman's Wages.
Supposedly the 13½d were the value set in a proclamation of James I as equivalent to a Scottish mark. (This is not far off from my suggestion about 13½d being a standardization of some commpodity price.) But Norman Yamada has pointed out that Dr. Zachary Grey, in his annotation of Butler's Hudibras (1744) speculated that the “hangman's wages” were themselves an allusion to the Halifax law:
I cannot really say, whence that Sum was called Hangman's Wages, unless in Allusion to the Halifax Law, or the customary Law of the Forest of Hardwick…