Question

In J.D. Slinger's "The Catcher In The Rye", he entails Holden writing Stradlater's composition about the baseball mitt that belonged to his younger brother, Allie who died several years ago.

Answer 1

My questions are:
1) How is Allie's mitt symbolic? (I put that it was something to remember Allie by, but what more is there?

2) How is Allie's death symbolic? (In a figurative sense.)