Doh is the first note of the sol-fa scale that singers use — the home note. The eight notes go doh, ray, me, fah, soh, lah, te, and then doh again, higher, which is why people say it is the first note or the eighth. In many books you will see it spelled do; doh is the English spelling, common in British teaching. Tunes often feel “finished” when they come back to doh. In Taiwan you already know this as Do, the 1 in 簡譜. Some classrooms keep Do as C all the time; others move it when the key changes. Same idea, two classroom habits. This is not the cartoon cry “D’oh!” — different spelling, different meaning.