, or occasionally even on more formal occasions like a recording session
, use is often made of a head arrangement
Rather than an arrangement in sheet music form, this is slang for just working out a rough arrangement among the musicians in question. For example when blues musicians are jamming together someone might say 'It's a twelve bar blues in E, boogie bassline, twice through the changes, guitar solo, one more verse and end'. This would be an example of a head arrangement.
However, this technique can often become vastly more complicated. When listening to the Pet Sounds Sessions box set by the Beach Boys, one can hear Brian Wilson effectively orchestrating for 13 or 14 instruments in this manner - handing out a basic chord sheet, showing them their parts, then calling out alterations on the fly.
Frank Zappa often also used to 'conduct' his bands onstage, rearranging and even writing songs with hand signals. This would be another example of this idea, but taken to its furthest extreme, and used with a very tight, well-rehearsed group.