What is the origin of the phrase "fits to a T"?

Question

Does anyone know?

Answer 1

Sorry if I seem too stuck on the details below, but that DOES seem to be in the spirit of the expression. And most if not all of those details are rather certain.

First, T-square is by not means a possible explanation, because the T-square itself was invented long after the expression was in use!

Now let's take a careful look at the almost certain explanation -- it is a shortened form of "to a tittle". This expression in use in English by the early 17th century, with the meaning "to the smallest detail" (the variation "to a T" or "to a tee" appears by the late 17th century).

Do note that the word is "tittle" NOT "title". The word "tittle" comes from the Latin word for a diacritical mark. It is RELATED to the word "title", but they are NOT the same word.
http://www.thefreedictionary.com/tittle
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About the BIBLICAL origin of the expression. There are a number of details that people often are a bit off on when they try to lay it out. (This is quite understandable since most of the people relaying the stories are not trained in Biblical Greek and Hebrew.)

A number of people correctly trace this expression back to John Wycliffe 14th century English translation of the Latin Bible. In Matthew 5:18, where the Latin has the word "apex" (the original word in the Greek literally means "horn"). Wycliffe chose the word "tittle", thus referring to a tiny pen mark that distinguished a letter.

It was a good choice. In this verse Jesus refers to very small marks on the top of certain Hebrew letters that distinguished them from very similar Hebrew letters, rather like the small stroke that distinguishes our capital Q from a capital O or G from C. (Some modern translations use expressions like "least stroke of a pen" to convey the idea.)
http://www.randomhouse.com/wotd/index.pperl?date=19980629

"Tittle" in this verse is is the second member of a pair, the now familiar "jot and tittle". The term "jot" renders Greek "iota" -- the name of the small Greek letter "i", though Jesus probably was speaking of the tiny equivalent HEBREW letter "yod".
http://www.randomhouse.com/wotd/index.pperl?date=20000223

For what it's worth -- some incorrectly try to connect these features with the GREEK alphabet. This is an understandable mistake, since the New Testament was indeed written in Greek. But to say that Jesus' POINT here is based on the Greek alphabet is highly unlikely. This becomes clear once you realize that the "lower case" letters of the Greek alphabet had not yet been invented. The "iota" was not so obviously "small", while the Hebrew "yod" in use at the time (same form as you would find in Hebrew Bibles today) is CLEARLY the smallest letter of that alphabet.

Even more important to the HEBREW explanation: Jesus whole POINT here has to do with the Hebrew Bible (called "the Law", which may refer just to the books of Moses --the first five books-- but often was used to refer to the WHOLE Hebrew Bible). He was NOT making a point about Greek translations of the Bible!
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"Jot" and "tittle" continued to be used in later the 16th century English Bible translations (with this material, from the New Testament, now being translated-- beginning William Tyndale in the 1520s-- directly from the original Greek New Testament). Many still know these terms from the King James (1611)** translation -- "Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled."
http://www.blueletterbible.org/kjv/Mat/Mat005.html#18

**By the way, the first documented use of "to a tittle" in this sense comes from 1607, so it is NOT based on the KJV, but on earlier English translations (probably most directly from the Geneva Bible, which was by far the most popular version at that time).

So, based on this Biblical "tittle" to refer to tiny details "to a tittle" --and the various expressions based on it-- was used to refer to precision (that is 'to the smallest detail').

see also:
http://worldwidewords.org/qa/qa-toa2.htm

Answer 2

Here are two theories.

The allusion here is said to be with a T square.
This piece of apparatus is so accurate that a precise right angle fits it perfectly.

However neat this suggestion is, there is another possible origin, based on the fact that the saying was in use in the 17th century, before the T square was invented.

This one suggests that the T stands for "Title", a minute and precisely positioned pen stroke or printer's mark.
A tiny brushstroke was all that distinguished the Hebrew letter "dalet" from "resh".
"Title" was the word chosen by Wycliffe to translate references to this tiny difference in his version of the New Testament.
Thus the mark was perfectly suited to its task.

Answer 3

Maybe it alludes to a T-square, used in construction, meaning precisely aligned.