The first இலக்கிய வகை type: a native Tamil word, used since ancient times in its plain, unaltered form - understood directly by anyone, learned or not. It splits by whether the root is a noun or a verb.

மரம், கல், மண், மனிதன், ஆடு - words a 5-year-old and a Tamil scholar understand identically, with no special training needed. That plainness is exactly what இயற்சொல் means.

மரம், கல், மண், மனிதன், ஆடு.