Its actually spelled trading "fores" and its a common medical term for a person with an amputated forearm who is attempting to have another person's forearm surgically attached to his stub.
Pretty please, with sugar on top, clean the f***ing car.
I love pulp...
Someone told me there's a girl out there
With love in her eyes and flowers in her hair...

I will find this girl.

Google. 2 minutes. C'mon it's the internet for Pete's sake.

Trading Fours on Urban Dictionary
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Trading fours usually means that two soloists 'trade' four bars each. So one of them improvises for four bars and then the other improvises for four bars, and so on, until the latter ends the 'duet'. Any instruments can trade fours, but it is commonly done either between two lead instruments (saxophone, trumpet, etc.) or between a lead instrument and drums.
isnt it also called 'trading fronts', im not sure my jazz band leader was telling as what to do as we played so I didnt actually hear what he called. I guess it could be like soloing is comeing to the front so as you trade solos you trade fronts?
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"The Blues are a simple music and I'm a simple man. But the Blues aren't a science, the Blues can't be broken down like a mathematics. The Blues are a mystery, and mysteries are never as simple as they look" - BB King