Three families of consonants

வல்லினம், மெல்லினம், இடையினம்

The eighteen consonants are traditionally sorted into three groups of six, by how hard or soft they sound.

வல்லினம் - Hard consonants

க ச ட த ப ற

மெல்லினம் - Soft (nasal) consonants

ங ஞ ண ந ம ன

இடையினம் - Medium consonants

ய ர ல வ ழ ள

This grouping is not decoration - Tamil grammar rules about which letters may follow which depend on these three families.