This grapheme is appended to the right side of a consonant, and changes the syllable's vowel sound from /a/ to /am/.
It can also change into a homorganic nasal before a plosive or affricate e.g. as /ŋ/ in സംഗീതം but not before other consonants where it stays as /m/ e.g. അംശം, സംരംഭം.
