The music of this game gradually develops whether you believe it or not.
Preparation
If there are many players, e.g. 7+, this game will require free movement around the playing area. If there are immobile instruments, players with these should be set in proximity to each other with eye contact possible. The game might be hard to arrange to play with many immobile instruments.
You will need pen and paper to play this game (or have a group with good memory).
Setup
As a group agree about five "stages of musical development" and write them down. What you look for are five brief descriptions (prompts, events) that fit your skill and musical preferences. The best order for defining the stages is: 1 (starting stage — something very simple to play), 5 (final stage — the most involved type of musical playing that you'll define), 3 (something in between in complexity), 2, 4.
The example below is intentionally exaggerated:
1) Silence
2) Letter 's' spoken
3) Simple and slow sounds of your instrument
4) Soft rock
5) Symphonic death metal
This list assumes that players have an idea of two mentioned musical genres and can improvise on their instruments something that will be reminiscent of these styles.
Note that outer levels (1 and 5) will most probably be used the most, so they should give players some freedom and also let them easily stay on that level for an extended amount of time in comparison to the stages in between.
At the end some players will be able to silence others, the default mode for it is a tap on the shoulder, but you may need to agree on a gesture or phrase, depending on your distances, mobility, and safety preferences.
Game start
All together play the musical instruction that was assigned for level 1.
Gameplay instructions
Use eye contact or walk around to meet with other players in pairs. When you meet in a pair, stop playing music for a moment and have a quick round of rock-paper-scissors.
If you win, upgrades to a higher stage of "musical evolution" (from 1 to 2 and so forth) and change the played music to the one according description of your new level. If you lose, downgrade, unless at level 1 or at level 5. Ties mean that both players advance. Always follow the musical instruction for your current level.
Note that in each round you should spend more time playing music than dueling.
Game end
If you reach the final step of the evolution, don't downgrade anymore. Now you have an option to end the game by tagging-out other players. You can continue to play, and in such case continue to engage in 'duels' with other players but stay at your final musical instruction regardless of the duel result. Once you start tagging out, you can't duel anymore.
The game might end soon after the first player escapes the evolution or it may continue until all players join at the final level, and it will last until someone wants to end it and starts to tag out other players.
Gamemaster's notes
You might need to agree in advance on exact procedure for rock-paper-scissors.
As always try to make everything musical, but here especially together within a pair that you just engaged in a duel (play a duet).
This is an adaptation of a game from improvisational theatre. Here as untied.
Please, do not set a Nirvana piece at the highest level of development ;) (this joke is made here so you don't have to).
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Finish tagging (_game) (regular library entries)
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