trade fours
v. phr.— «We shared a too brief lunch during which both father and son in the special parlance of jazz musicians “traded fours,” with a rising crescendo of memories of all that Herb Pomeroy, trumpeter and inspiring teacher, had given to others of music born in the very soul of humankind.» —“‘Trading fours’ in memory of trumpeter Herb Pomeroy” by Bill Plante Daily News (Newburyport, Massachusetts) Aug. 24, 2007. (source: Double-Tongued Dictionary)
Further reading
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Related
Traditionally, musicians pass the solos around the band by choruses — each soloist might play one or two choruses, followed by the next player. “Trading fours” refers to a more personal interchange between musicians: each soloist plays only four bars, then the next one picks up the solo and either complements or contrasts the first with his four. Then the solo passes back to the first musician. It’s often used when two skilled players do a song together, creating a “conversation” between them.