T'i to behave as a younger brother is a verbal sense developed from the original sound and signification of the character read ti⁴ = younger brother. For convenience' sake the former is sometimes written 悌, with 心 hsin heart as radical.

Ti has been regarded as an altered form of 第 ti order, series, from which the sense of younger brother has been developed. It is also said to be the picture of a thong encircling a faggot. In colloquial, 兄弟 hsiung ti means younger brother, and 弟兄 ti hsiung means brothers.