In Theodore Roethke's "My Papa's Waltz," the speaker mentions the buckle in the third stanza of the poem.  In the context of the poem, the buckle is a helpful indicator of the speaker's height, and the reader infers that if the speaker's ear hits the buckle, then he must be relatively short and hence young.  Further, the speaker says that he hits the buckle every time his father misses a step in their "dance," indicating that the father is drunkenly dancing.

Some readers interpret the buckle as an indicator that the father is abusive to the son.  In times when disciplining children was common practice, parents might use a belt when punishing their children.  So some readers see the buckle as a symbol for this type of punishment taken a step too far when the father is drunk.