ANSWER: This one is getting a little long in the tooth, like its users (such as me), who can remember when beer always came in bottles fitted with caps that needed a special tool to open them (though the more macho or foolish would show off by opening them with their teeth; gripping bottle tops in a vice between door and doorframe was a trick for emergencies, but only when the door belonged to somebody else). The standard bottle opener was made of metal, with a roughly round, oval or triangular open shape at one end to grip the cap.

The shape of the business end of the tool reminded people of the often ornate handles to big, old-fashioned door keys. The link with churches in particular was surely because in the experience of most people such big keys opened church doors. It’s also more than probable that an irreverent joke was attached as well, in that drinking beer was an unchurchly thing to do.

Before the messages start to be written, let me rapidly move on to a further stage in the development of the term. As you say, the phrase church key is only recorded in print from 1951, though there is much anecdotal evidence to suggest it is rather older in the spoken language, perhaps from the late 1930s. This was around the time at which beer began to be sold in cans rather than bottles. These early cans also needed a tool to open them, since the pull tabs of today were not to be introduced until about 1962. The tool was a stout flat strip of metal with a sharp point, which you pressed into the top of the can to puncture a triangular hole (two were needed on opposite sides, I recall, to let air in so the beer would flow easily). By an obvious analogy, these also came to be called church keys, even though they were a completely different shape.

Another explanation comes from Mr. J. R. Oberhofer, an old-time brewery worker:’The expression church key is old in the brewing business,’ he wrote. ‘I worked in a brewery for about 35 years and everybody carried a bottle opener or church key, perhaps so called because it looked like the top end of the kind of heavy ornate key used to unlock church doors.

I am enclosing an old relic that is about 50 years old. It’s made of cast iron and from its weight and appearance, you can see its resemblance to a church door key. With the coming of cans in the brewing business, the bottle opener gave way to the can opener that makes the triangular marks – but the name church key was simply transferred to the new device.’ Mr. Oberhofer actually did send the cast iron bottle opener, and the evidence seems to me entirely persuasive.

(The cap on these beer bottles, by the way, is a crown cork, named after a fanciful view of the ring of crinkled points around the edge of the metal closure before it was clamped on the neck of the bottle. It was invented in 1898 by William Painter, and his firm, the Crown Cork and Seal Company of Baltimore, is still very much around, though these days it spends most of its time making aluminium cans and other packaging products)

If you have any beer CANS that took an opener (or churchkey) to open,drop me an email.