Blackwell’s latest ‘Bogof’ offer is advertised so prominently in the windows of its Broad Street shop that no one passing is likely to miss details of it. I wonder how many of those studying the advertisements, however, will have raised an eyebrow, tut-tutted or howled with rage – according to their strength of feeling on matters to do with the use of language – over what is said in small print in the corner of the big pink signs.

The asterisk beside the words “Buy one get one free” links to the explanatory statement: “Cheapest item free.” For some people this will simply signal a pretty good deal in prospect; for others it will mean a regrettable lapse for a company that styles itself “The Knowledge Retailer”.

The error – which is repeated on thousands of stickers attached to the books themselves – is in the word ‘cheapest’, a superlative. Where only two items are concerned – as they are here – the comparative ‘cheaper’ is what is required.

The matter is eloquently summed up by the great Oxford lexicographer Robert Burchfield in The New Fowler’s Modern English Usage, published in 1996. He wrote: “In general it is a sound rule that confines the use of comparative forms of an adjective to contexts in which two entities are being compared, [note the Oxford comma] and reserves superlative forms for comparison of three entities or more.”

There are writers who break this rule, including William Shakespeare who is entitled to break any rule with impunity. He, of course, is the man who gave us the memorable double superlative of Mark Antony’s “most unkindest cut”. In The Merchant of Venice he supplies the Prince of Morocco with the line: “to prove whose blood is reddest, his or mine.”

Burchfield also cites the examples of Daniel Defoe (“whose God is strongest, thine or mine”) and Jane Austen (“She was the youngest of the two daughters”).

With a certain amount of glee, I suspect, he also quotes Henry Watson Fowler, he of the original Modern English Usage, who wrote there: “Dinghy, dingey. The first is best.”

Summing up the matter, Burchfield concludes: “Clearly there is a ragged edge at the rim of any strict rule, but the general pattern should normally be adhered to, leaving exceptions only to the truly great or to literary or linguistic licence.”

I hardly think that Blackwell qualifies under any (either?) of these grounds.

Readers will note that Oxford’s world-famous book supplier these days answers to Blackwell rather than Blackwell’s, as it referred to itself for years. There was a time a decade or so ago when it started to call itself Blackwells. This did not go down well with its then boss, since the change coincided with one of those period fits of public concern over misuse of the apostrophe.

We are going through another one of these at present, sparked by Birmingham City Council’s decision to eliminate this punctuation mark from signs throughout its area. This has provoked a great deal of criticism, though it seems to me eminently sensible.

In Oxford, if we followed suit, we could eliminate at a stroke all that confusion over St Giles’s, St Clements, St Ebbe’s and St Aldates’.

Which of these, if any, are right? Don’t ask me; I always have to look them up.

We have, in fact, been dropping apostrophes when it suits us for many years. To give but one example: do you think that the very first word of this article – had it been written in the days when Blackwell was Blackwell’s – would have appeared (as surely it should have done) as Blackwell’s’s.

You have it in one.