Outside intu Chapelfield in Norwich there's a sign saying that the Mall is open 'til 7pm. This is an interesting spelling – the normal way of writing 'til is till. So why would it occur to anybody to write it with only one l, and with an apostrophe before the t?

The usual reason for using this kind of apostrophe in written English is to show that something that used to be there isn't there anymore, as with the n't in don't and isn't. In these cases, n't is a reduced form of not – don't derives from do+not. It's a little bit more complicated with won't and shan't, which do not come directly from wo+not and sha+not. The n't part of don't and isn't is a weak form which cannot occur on its own: it isn't possible to say 'I don't know whether to go or n't'. The 's in it's and the 're in we're are similarly reduced forms which cannot appear on their own.

But 'til is not like that. It can occur on its own, as it does on the sign at the shopping centre. And nothing has been left out. 'Til isn't a reduced form of anything – it's just a misspelling of till. The misspelling is based on a total misapprehension, namely that till is a reduced form of until (which is nowadays spelt with one l for no particular reason). In fact, till is not a shortened form of until. The opposite is true – until is an expanded form of till!

Till was originally the Old Norse word corresponding to Old English to. In the modern Scandinavian languages, it's still the ordinary word for to. In Norwegian and Danish it's spelt til, while the Swedes spell it till. In parts of the North of England which were very heavily Scandinavianised through Viking settlement, till is still used in local dialects to mean 'to' in many of its senses, as in a quarter till eight – some Americans say that too. But in most other forms of English, till just means 'as far as' or 'up to'.

So where did the un- bit in until come from, then? The answer is that it was an Old Norse form which has now disappeared. Un- originally meant 'as far as'. So un-till was simply a stronger, reinforced way of saying till. It would have been the equivalent of modern English 'right up to'.