Symbolism, Imagery, Allegory
Not all the characters in Feathers communicate out lout—and because Sean, Frannie's older brother, is deaf, she's grown up with sign language as one of her primary methods of communication. She tells us:
By the time I was born, Sean was two and a half years old. I grew up learning how to speak and sign. (8.42)
A lot of people think that sign language is strange or out of place. Just look at what happens when Jesus Boy signs at Frannie—the other kids think that he's a straight-up weirdo for knowing sign language even though he's not deaf:
"Are those real signs?" Samantha asked me. "Or is he just being jive and faking it?"
"It's real," I said. "He's saying he's not deaf, that's all."
"Well, how does he know that?" (2.54-56)
But to Frannie, sign language isn't weird or out of place in her world—it's just another perfectly ordinary way of communication. Symbolically, then, sign language ties into the idea that just because something is different doesn't mean that it's lesser or that it should be shunned. In a way, the use of sign language comes to represent how the other kids feel about Jesus Boy as much as it represents how kids feel about Sean, and how when it comes to both of them, other people are wrong in their assessments.
Sean and Jesus Boy may be different, but that doesn't mean that they're not normal kids, just like how sign language is just another totally valid way of communicating.