A hand (又) held against a measuring rule (寸): checking something against the standard. What matches the standard is correct; facing something to compare it gave 'toward'.
对 is the Swiss-army knife of Mandarin: 对不起 sorry, 对了 'oh right!', 面对 to face. Agree with someone? Just say 对对对 — the triple 对 is practically punctuation in Chinese conversation.
A HAND against the RULER — checking it's CORRECT.
对 also means 'a pair' — 一对 couple. Correctness and coupling share one character: things that match.