Included in Play in a band set/collection.
Share the spectrum and find creative ways for swapping it's areas.
Setup
Music for this game might be performed by many people, but there will be three gamers who will interact by the rules.
First, suitably to the possibilities of players' instruments, agree on an aspect of music over which you will play. For first games maybe take pitch or rhythm. You need players to be able to use your chosen aspect.
Now define three distinguished states of the chosen aspect. For example if your aspect is pitch, go for high, middle and low ranges of the instrument; with rhythm you might settle for meters of 3/4, 4/4 and 5/4 (many more options here) or regular, half- and double-time (easier).
If you play over pitch, have pitched instruments, but also non-minimum range. If you want to add accompanists, additional music might in this case be provided by some percussion. Rhythm option of the game might be accompanied by long tones (drones) of strings or synths etc.
Make sure that players will be able to clearly hear each other — for the first try, it would be best to stay with only 3 gamers, without distractions.
Gameplay instructions
Your aim is to stay in a different state than two other players, at any given time. But even if you are alone, do not stay in one state for long. Listen carefully to other players and recognize in what states they are, fill the empty state-spot when necessary while keeping everything musical and coherent.
Game end
Any player can end the piece just by stopping playing.
You will need to come up with different ending (hand gesture maybe?) if you decide to play for example over aspects of notes density or volume (as scarce notes or silence might be mistook for a signal to finish the game).
Gamemasters's notes
This free-form can range from natural and obvious musical activity to weird and hard adventure, so it's not easy to set for one difficulty level for it.
The game is suitable for electronic instruments — you may even end up having a single knob for the aspect you play over.
The game is here as untied. It is directly related to "Sitting, Standing, Laying" game from Improvisational theatre.
Example
The example above is played on the density of notes.
Editor's notes
Photo used for decoration: "Improv game" by Isabelle Adam (CC BY-NC-ND 2.0)
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