Why was letter T chosen in T-joins in Graph Theory

Question

In one of my master's courses, Advanced Graph Theory, we studied T-joins and their applications.

I asked my teacher why was letter T chosen in T-joins' name and he didn't know. I guess there's probably an explanation or a word behind this T, what is it ?

Wikipedia's T-joins page

EDIT : (Misha Lavrov's comment) We are using the letter T in "-join" to refer to the set of all odd-degree vertices. My question is "why are we using the letter to represent this set"?

Answer 1

The term is used at least as far back as this paper by Seymour with no explanation. I see two possibilities:

1. There is no reason. After the letter $S$, $T$ is a candidate for the most natural letter to use for a set, and maybe there was a reason not to use $S$, or maybe Seymour just didn't like $S$. Afterwards, everyone followed the existing convention.
2. The shape of the letter $T$ is meant to be evocative of the odd-degree vertices that are elements of $T$ in the case of the postman problem (the center of the top bar of the $T$ looks like it's a vertex of degree $3$).

I think the first possibility is likelier. The same paper of Seymour also uses $T$ for the subset of the vertex set used in Tutte's theorem. (For this, it is now conventional to use $U$.)