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"Until" should always be written with one "l" and "till" with two "ls". Do you agree?
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I and the American Heritage also like 'til:

'USAGE NOTE: Till and until are generally interchangeable in both writing and speech, though as the first word in a sentence until is usually preferred: Until you get that paper written, don't even think about going to the movies.Till is actually the older word, with until having been formed by the addition to it of the prefix un–, meaning “up to.” In the 18th century the spelling 'till became fashionable, as if till were a shortened form of until. Although 'till is now nonstandard, 'til is sometimes used in this way and is considered acceptable, though it is etymologically incorrect.'
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Mister MicawberI and the American Heritage also like 'til:

'USAGE NOTE: Till and until are generally interchangeable in both writing and speech, though as the first word in a sentence until is usually preferred: Until you get that paper written, don't even think about going to the movies.Till is actually the older word, with until having been formed by the addition to it of the prefix un–, meaning “up to.” In the 18th century the spelling 'till became fashionable, as if till were a shortened form of until. Although 'till is now nonstandard, 'til is sometimes used in this way and is considered acceptable, though it is etymologically incorrect.'

So "till" is actually an abbreviation of "till" and not of "until", right?

Apparently "until" was originally written as "untill".
Yes, I agree. I recently discovered that "till" and "until" are in fact two separate words, "till" is not an abbreviation of "until", and in fact "till" existed in the English language before the word "until" did. Here's a link to an interesting explanation:

http://www.worldwidewords.org/qa/qa-unt1.htm