A method of visually presenting drum and percussion notes in a sequencer or drum machine. Vertical lines on the grid represent "beats", in whatever musical unit of time is currently being used for the beat (most often quarter or eighth notes). Each drum sound is assigned to one horizontal line. Symbols are placed at the intersection of horizontal and vertical lines to indicate drum strikes at the specified times. Often, different sizes or types of symbols are used to indicate features like velocity/accent, rolls, flams, etc.
The concept has been around for centuries; classical composers who wrote in conventional sheet music often used lines of the staff to represent individual drums and unpitched percussion instruments. Probably the first electronic implementation of the concept was in the circa-1984 Simmons SDS-6 sequencer, which used a massive array of red LEDs to represent the grid points. Roland introduced a more practical concept the following year with the TR-707 drum machine, which displayed the grid on an LCD screen.