Rhythm

What it is. Actions that only count on the beat. A clock ticks in musical time; you press inside a window around each beat, and the world answers in tempo. Whether it’s tap-the-note, move-on-beat, or fight-on-beat, the game is a legality filter over time: right thing, right frame.

Player fantasy / why it’s fun. “I’m locked into the groove and the game is moving with me.” The pull is embodied precision — the satisfaction of a press that lands exactly, the whole system pulsing in time. Fun is tight but fair: a window narrow enough to demand mastery, honest enough that a miss is always yours.

Pillars

The beat is sim time. Pick a BPM so one beat is an integer frame count. The sim advances on the beat counter; audio is an observer that schedules sound off that counter. Never the reverse — audio-driven timing is unprovable and drifts.
Fairness is frame-exact. Three assertions define the whole feel: the input is accepted at the window edge, refused at edge+1, and hammering acts once. If those three hold, the game is fair; if any fails, it’s a lie you can feel.
Rhythm is a filter over a turn-based game. Underneath the beat is a discrete turn system; the rhythm layer is a thin input-legality gate (~30 lines over a roguelike). Design the turn game first, then constrain when moves are legal.

The loop stack

Layer	The beat
Moment	Hit the window on the beat → the action fires in tempo.
Encounter	A phrase/pattern: a run of beats to nail without breaking the chain.
Session	A track/level: sustain accuracy from start to finish.
Meta	Song select, difficulty tiers, combo/score mastery, new charts.

Essential systems

System	Why this genre needs it
`pattern-juice-choreography`	Feel IS choreography timed to the beat; every hit’s flash/pop must land on the frame the sim fired.
`system-grace`	The hit window and input buffer are a grace system — accepted-inside / refused-outside, proven to the frame.
`system-difficulty-and-dda`	Difficulty = window width, note density, and tempo; the tuning dials of “tight but fair”.
`system-onboarding`	Teach the window by doing on a slow, forgiving track before narrowing it.
`system-mastery-curve`	The skill ceiling is timing precision; charts must reward tighter play with higher scores.
`system-procgen-design`	Charts as content; generate/validate patterns that stay hittable at tempo.

Content & difficulty model

BPM is chosen, not found. Solve for a BPM where a beat is an integer number of frames; every chart is authored on that grid. A non-integer beat makes the window jitter and the three frame assertions impossible to state cleanly.
Window honesty is the whole verify. Assert the edge-in, edge+1-out, and hammer-acts-once cases frame-exactly. These three are the genre’s mechanical truth — state them before charting a single note.
Foes are frozen between beats. If the underlying turn game has enemies, prove they act only on their beats — no between-beat movement — or the rhythm read breaks.
Difficulty ramps three dials. Narrow the window, raise the density, lift the tempo — independently, so a chart can be fast-but-forgiving or slow-but-strict. Never make a chart “hard” by making the window dishonest.
Hash-identical replay. A beat-perfect bot must clear, and the same inputs must replay to an identical world.hash() — the beat clock makes the whole sim reproducible.

Signature-mechanic seeds

Rhythm but it’s a roguelike — every step is a beat — move, attack, and dodge only on the pulse; the dungeon is a turn game with a metronome (structure; the literal “filter over a turn-based game”).
Rhythm but you’re clearing a match-3 board on tempo — swaps only register on the beat, cascades resolve in time (mechanic-swap; fuses genre-match3‘s cascade with the beat clock).
Rhythm but the beat is a duel — attacks land only on-beat against a telegraphing foe; parries are windows (mechanic-swap; a fighting game gated by tempo).
Rhythm but one button, one lane, escalating tempo — a hard constraint that makes pure timing the entire skill (constraint; the anchor-tetris “one perfect verb” read).
Rhythm but missing isn’t failing, it’s souring — off-beat presses detune the music instead of ending the run (tonal; forgiveness as feel, pairs with system-grace).

Common pitfalls

Audio-driven timing. Scheduling the sim off sound makes it drift and unprovable; the beat counter drives, audio observes. This inversion is the genre’s cardinal sin.
Non-integer beats. A beat that isn’t a whole frame count makes windows jitter and the fairness assertions unstateable. Choose the BPM to fix this.
Dishonest windows. A window that’s secretly wider/narrower than shown, or that accepts a hammered double-press, feels like a cheat even when the player can’t name it.
Between-beat enemy motion. Foes that drift off the pulse destroy the read; freeze them between beats and prove it.
Charts before the clock. Authoring notes before the frame-exact window is proven bakes unfairness into every level. Prove the three assertions first.

Anchors

anchor-tetris — the perfectly-tuned single verb and pure endless mastery; the reference for the one-button, escalating-tempo bend.
anchor-peggle — maximal juice on a precisely-timed input; the choreography-on-hit feel the rhythm genre lives or dies on.

Verify

Beat-perfect bot clears; window honest to the frame; foes provably frozen between beats; hash-identical replay → design/FUN.md §18 · Rhythm. Design the beat grid and turn game here; prove the three frame assertions there.

Composes with

pattern-juice-choreography — the on-beat feel; every hit choreographed to sim time.
system-grace — the hit window as a frame-exact grace system.
system-difficulty-and-dda — window/density/tempo as the difficulty dials.