A drum roll (or roll for short) is a technique used by percussionists to produce a sustained sound for the duration of a written note.

All drum figures are based upon three fundamental beats, technically called roll, single stroke, and flam...Sustentation is accomplished upon wind instruments by blowing into the instrument; it is accomplished upon the violin and the allied instruments by drawing the bow across the string; it is accomplished upon the drum and allied percussion instruments by the roll.

THE SNARE DRUM ROLL.

The roll consists of an even reiteration of beats sufficiently rapid to prohibit rhythmic analysis. To produce an impression of sustentation, these beats must be absolutely even both in power and in sequence. Uneven beats in a roll destroy the impression of sustentation. Evenness is then the primary quality to strive for in roll; speed is the secondary quality to strive for.

There are two possible ways of producing an absolutely even sequence: (1) hand alternation of single stroke and (2) hand alternation of double strokes...The snare drum roll is produced by hand alternation of double strokes.

The "open roll" is produced by [initially] slow hand alternation. Two strokes in each hand alternately are produced by wrist movement and each beat should follow its predecessor in clock-like precision.