It's to do with the origin of X to denote Mild Ale. Imagination and conjecture seem to have been used in place of research and reason in coming up with the various theories expounded below.
"TOWN AND COUNTY GOSSIP.
A few weeks back the "Daily Chronicle" made a reference to the sign of X which by long custom has become the mark universally employed by the brewers of this country to distinguish a certain popular type of ale. The suggestion was made that the sign originated among the monks of Burton, who are said to have marked their casks with the sign of the cross as an indication of the origin and superiority of their ales. Mr. Matthew J. Cannon, F.C.S., in the "Brewers' Gazette," rather inclined to doubt this theory. He says:—"It is doubtful whether the theory that the monks of Burton employed the Christian emblem to mark their barrels would prove entirely sound on analysis. We know that in early English times the true cross form and the crossed diagonals both did duty for the same letter. But among the monks, who, far from being illiterate, were expert in the art of writing, the transition from one form to the other is not satisfactorily explained. If we are led astray by mere similarity in form or corruption of sign, other explanations of the derivation of X could be I found with facility. In mediaeval times the citizens of London were much exercised regarding the quality their ales. What would be more natural the brewers of those days than that they should demonstrate the quality of their ales by marking their casks with a representation of Walworth's dagger, the chief device upon the civic shield. Although these explanations may be incorrect, they convey a hint that the sign of X originated in the Middle Ages. If the antiquarian pursued his studies with assiduity, it is not unlikely that he would discover the origin of the sign among the wealth of writings which describe the domestic life of the early civilised peoples of antiquity."
"The brewing of beer did not originate in Burton, it was brewed throughout the land early Saxon times; indeed, the brewing of barley wine ante-dates the Christian era. There was corn in Egypt, and beer too, for the office of brewmaster was an honoured position in the King's household. Even in remote Babylonia it is yet to be proved that barley wine was unknown, nay, may have graced the board at Belshazzar's feast. The brewery clerk of those days might have found it easy to make two crossed impressions of his stylus upon the clay tablets, which then did duty as an invoice, for every measure of beer delivered. Or, in later days, when cuneiform gave place to the pictorial, the head of an ox, the aleph, the alpha, or beginning, would be a fitting symbol. The letters A and X are so far apart to-day that it is a purely fanciful speculation to attempt to trace any resemblance between them. But accomplished palaeographists are able to trace the transition from one form to another even in the most unlike symbols. Herein is ample field for study by the scientific inquirer into the antiquity of the mysterious sign of X. But it is probable that with much learning he must be led astray. After all, the sign may be comparatively modern, having its origin some common-place practice in the cellar, simple marks placed upon the casks showing different stages of the beer before it was ready to send out. But whatever explanation may be offered as to its derivation, it will still remain a puzzle why this distinctive symbol has become the representative mark of mild ale."
Derby Daily Telegraph - Tuesday 28 May 1912, page 2.
Two things are quite important here:
- I've never found X's used for beer names before 1800;
- X was used for other types of beer than Mild in the 19th century.
I'm pretty sure that X wasn't used until several centuries after the last monks had been driven from Burton. And as to it originating in London in reference to a dagger on the city's coat of arms, well, that's such obvious rubbish, I won't waste any more words on it.
Unless, of course, Mr. Cannon is taking the piss. I'm tempted to believe that when he talks about it possibly originating in Babylon. Based on what evidence? That they had writing of some sort.
I'm quite shocked by the final sentences. Yes, he said the term might be of comparatively recent origin. But as to it being a mark showing how close the beer was to being ready, surely someone in the industry must have known better. Afterall, there were still beers called XX, XXX and XXXX when he wrote the piece. It's obvious that the X's are an indication of strength, not readiness.
What is the origin of X's? I don't know for certain, but I suspect it comes from the period before 1830 where there were two tax categories for beer, Table Beer and Strong Beer. I know that barrels of Table Beer had (on orders of the Excise) to be marked with a T. X seems to have been the mark for Strong Beer. It could possibly come from the rate of tax - 10 shillings per barrel - on Strong Beer. Or it could just be an easy mark to make with a piece of chalk.
As I keep telling you, nothing's new. Not even crap beer history.