Part of Family 6 (Colour & effects) — masking part of the screen. The PPU has two hardware windows: rectangular (or, driven per scanline, any-shaped) regions that gate what each layer and colour-math shows. Spotlights, portholes, iris wipes, "only darken inside this shape" — all windows.

Not the same as panel (tile boxes) — these are the PPU's masking registers.

The ladder

Rung	Example	Developer question
6f.1	window	How do I mask part of the screen with one window?
6f.2	window_multi_hdma	How do I shape both windows per scanline?
6f.3	transparent_window	How do I combine a window with colour math (a darkened region)?

Climb from one static rectangle (6f.1) to two HDMA-shaped windows (6f.2) to a window gating colour math (6f.3).

The idea in one screen

Each window is a left/right edge pair (WH0/WH1, $2126–$2129); a layer or colour math reads a per-layer bit that says "apply me inside / outside the window." Set the edges once and you get a static rectangle. Rewrite the edges every scanline with HDMA and the two straight edges trace any curve — a circle, a wave, a shrinking iris. Point colour math at the window instead of a layer and you darken/tint only the shape.

Transparent Window -- Color Math + HDMA Window -- HDMA Triangle Masking Multi-window HDMA — both windows animated per scanline