Throughout history, using two bass drums has been a normal part of the drums for jazz bands, but recently many drummers, especially those in the hard rock and heavy medal genres have used dual bass drums. Since the 1980s it has been commonplace for drummers to use electronic drums either individually or as part of a standard drum set. Sometimes cowbells, gongs, tambourines, and other percussion instruments are also utilized in drum kits. A drummer may also have his own personal preferences in spite of those dictated by his genre, and therefore, creating a sound that is slightly different from every other performer in that genre. Some drummers also choose both snares and toms, and though they may not use them on every song, they become part of the drum kit to be utilized whenever needed or desired.
Though genre sometimes indicates the type of drums that are included within a kit, there is no hard and fast rule on it, In fact, even hard and heavy metal bands sometimes tone down a few songs on a CD, and thus the need does not exist for the harder sounds. Even some rock and roll bands from the past who had hard hitting drum sounds occasionally slowed it down through the use of just a bass or snare, allowing the guitars to carry most of the musical sound. After all, when you're talking about a ballad, you may not want the hard-hitting drum sound, but just a slow beat and occasional cymbal sound is all that is necessary.
The key to knowing what you need in your drum kit is in the type of music you will play, the venues where you will be performing, your budget, and your transportation resources. Personalize your kit based upon what you can transport and the size of the stages where you will perform rather than what you feel you should have or what you want. Even if you can afford it, it's senseless to buy something you can't use except for practice.
