Stable raster
The foundation trick that removes the jitter from a C64 raster interrupt so code lands on an exact cycle.
When a raster interrupt fires, the 6510 must first finish whatever instruction it is running, and since instructions take different numbers of cycles the interrupt arrives with a few cycles of random jitter. That is useless for effects that must write a register on one precise cycle.
The classic cure is a double interrupt: the first roughly positions, sets up a second interrupt one line later, then burns a known number of cycles with NOPs so the second fires cycle-exact. Described on the Commodore 64 as early as the mid-1980s, it is the bedrock beneath FLD, VSP, and side-border opening.