A bass drum is a large, heavy drum that produces a low-pitched (but untuned) "thump". It is used in orchestral music, marching music, and throughout 20th century popular music.
The bass drum is used to punctuate time. In marches it is used to keep the march even (marching bands march to the beat of the bass). A basic beat for rock and roll has the bass drum played on the first and third beats of a bar of common time, with the snare drum on the second and fourth "back beats".
An orchestral bass drum is quite large, about 36" in diameter, and is played with one or two large, padded mallets. In a drum kit, the bass drum is much smaller, most commonly 20" or 22" but sizes from 18" to 26" are quite normal, and is played using a pedal operated mallet. A pedal-operated bass drum is often called a kick drum.
In many forms of heavy metal music, particularly thrash metal, a second bass drum pedal is used, either operating a second beater on the same drum using an extended mechanism, or more traditionally simply playing a similar second bass drum, placed beside the first and tuned identically. Double bass drum techniques were pioneered by Gene Krupa in the 1950s and popularised in the 1960s by Ginger Baker of Cream and Keith Moon of the Who.
In many modern marching bands and drum corps, a set of multiple bass drums called tonal bass drums are used. A bass drum line typically consists of four to six tonal bass drums, though many smaller high schools only have enough players to use two or three drums, and some universities and drum corps have marched up to sixteen at once. The drums typically are between 18" and 32" in diameter, but some groups have used bass drums as small as 14" and as large as 36". Generally, tonal bass drums are tuned higher than kick drums or orchestral bass drums so complex rudimental passages can be heard clearly. Each player usually carries one drum, though in rare cases players carry two or three smaller drums at once. Skilled bass drum lines can execute complex rhythmic figures so convincingly that it sounds like the entire bass drum part is being played by a single musician.