An எழுத்து type for யாப்பு: a bare consonant with no vowel of its own - it carries no மாத்திரை by itself, but joins a குறில்/நெடில் to help form an அசை.

ஒற்று never stands alone in speech - try to say the sound alone, with no vowel before it, and you'll hear why it always leans on a குறில் or நெடில் for support.

மரம் - the "ம்" and "ர்" endings are ஒற்று.