What is the origin of the phrase "to the t"?

Answer 1

Actually, to be precise (which is fitting in this case!) the expression is "to a T".

Contrary to one of the most popular explanations it has NOTHING to do with the T-square (which was invented long after the expression).

This is a late 17th century variation of the expression "to a tittle", which was in use by the early 17th century, with the meaning "to the smallest detail."

The word "tittle" itself comes from the Latin word for a diacritical mark (and is related to the word "title").
http://www.thefreedictionary.com/tittle

But the key was John Wycliffe's translation of the Bible into English in the mid-14th century. Translating the Latin of Matthew 5:18 (which used the word "apex") he used "tittle". In the Greek the word used literally means "horn". By this, Jesus was probably referring to tiny marks on the top of certain Hebrew letters that distinguished them from very similar Hebrew letters (hence the modern paraphrase "least stroke of a pen").
http://www.randomhouse.com/wotd/index.pperl?date=19980629


Note that "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



"jot" and "tittle" continued to be used in later the 16th century English Bible translations (beginning William Tyndale in the 1520s). 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

Based on this Biblical "tittle" to refer to tiny details "to a tittle" was used to refer to precision (that is 'to the smallest detail').

Answer 2

Same as "to the 9's."