Five types of ullurai are described in the Tolkappiyam, an early treatise on grammar and poetics.
[2] Ilampuranar, an early mediaeval commentator, describes the constituents of each of the five types of ullurai.
Ilampuranar states that the key characteristic of ullurai is it functions as a literary device which causes the reader to perceive or understand something - a person, object or feeling - that is different from what the words of the poem describe.
[11] Modern commentators are divided on the nature of the relationship between ullurai and other literary techniques described in traditional treatises on Tamil poetics.
[12] Selby treats the purpose of ullurai as being the creation of iraicchi - a sense of recognition in readers, which leads them to understand the inner meaning of the poem.