It's an insignificant question I know, but Mat is always mat, or Lord Mat, or Mat Cauthon, I don't think anyone ever called him mattrim. How did Tuon know his full name was Mattrim?

Wild guess: Setalle Anan told her.

KoD, Ch. 36

 

"I fear you have mistaken me, Banner-General Karede." For the space of a sentence his voice became extremely resonant. "I am a gleeman, a position higher than court-bard to be sure, but no general. The man you want is Lord Matrim Cauthon." He made a small bow toward the young man, who was settling his flat-topped hat back on his head.

 

Tuon was there to hear that, and a bit later was the first time she used the name Matrim.

KoD, Ch. 36

 

"I fear you have mistaken me, Banner-General Karede." For the space of a sentence his voice became extremely resonant. "I am a gleeman, a position higher than court-bard to be sure, but no general. The man you want is Lord Matrim Cauthon." He made a small bow toward the young man, who was settling his flat-topped hat back on his head.

 

Tuon was there to hear that, and a bit later was the first time she used the name Matrim.

 

easy to overlook the obvious, thanks.