An எழுத்து type for யாப்பு: a long vowel letter, taking 2 மாத்திரை to pronounce - double a குறில்'s duration.

Say அ then ஆ back to back and you can physically feel the second one take twice as long - that felt difference is நெடில்'s whole definition.

ஆ, ஈ, ஊ - each exactly 2 மாத்திரை.