Symbolism, Imagery, Allegory
This one's a quickie.
You'll remember that when Katsa discovers that Po has lied about his Grace, she's pretty miffed. And when Po tries to explain, well, she stays miffed. Until… she kind of seems to start to get it. And one way we know she's starting to get it? Check this out:
"The mortar was cracking in the frame of the window. She pulled out a small chunk and rolled it between her fingers" (14.125).
That right there is a symbol. The cracking mortar in the window is a metaphor for what's going on with Katsa. A window frame has sharp angles; it's rigid and unyielding—just as Katsa seems rigid and unyielding with Po when she feels he has betrayed her trust. But as she begins to understand his motives and reasoning, her tough exterior begins to crack, just as the mortar has begun to crack in the window frame.
Nicely done, Ms. Cashore. Nicely done.