The primary purpose of a drummer is keeping time. The secondary purpose is laying the groundwork for changes in a song. This preparation is called a "fill." A fill is where the drummer foreshadows changes in the song, such as from a verse to the chorus, or the chorus to a bridge. Most musicians consider excessive filling irritating and a nuisance. Drummers such as Peter Erskine stress the concept of taste in fills -- use them only when necessary and conform them to the pattern and mood of the song.
Assuming 4/4 normal time, such as in most popular music, it is customary to have some kind of fill at least every 16 bars. In most popular music, 16 bars is the normal time for some type of transition, most commonly from the verse to the chorus of a song. A drummer prepares the listener --- and the other musicians --- for this transition with a fill. In many ways, a fill is a deliberate disruption in the normal rhythm of a song permitting the listener to prepare himself for a change.
Fills and Genre
The type of fill used depends on many factors. The type of music is one. Easy listening does not lend itself to flamboyant fills but usually some minor disruption in the last two quarter-notes of the last bar in the 16-bar cycle. Heavy metal relies on heavy cymbals and loud, powerful and fast playing. In this case, the fill can be more extended. Niko McBrain, of Iron Maiden in the early 1980s, pioneered the long and extended fill, which sometimes lasted two or more full bars. Traditional jazz does not normally consider fills in this systematic way but will often increase the use of the snare drum or toms as the verse moves into the chorus. A jazz drummer needs to listen closely to the soloist, since the "fill" in this case will mimic the actions of the solo. If a sax player gets louder, faster and more improvised as a solo progresses, a drummer imitates this, getting more active in response.
Fills and Tempo
Slower tempos often dispense with fills altogether or use them in a very subtle way. Many slow tempo fills are little more than a raised hi-hat on the last eighth note of the bar or the use of a tom rather than the snare on the last eighth note itself. Fast tempos imply more freedom for a drummer, especially in traditional jazz, and in this case, the fill is more elaborate and skillful. Steve Gadd, in the late 1980s, began to experiment with almost fill-less technique that puts the drums completely in the background as time keepers.
Fills and Instruments
Big bands demand more precise playing with less freedom for the drummer. A jazz trio demands a lighter touch that permits more subtle fills, including the use of brushes and cymbal bells, but much more freedom to innovate. The complexity of a song is also significant. Simple, straight-ahead blues songs imply far less filling, while bass-heavy rock or funk require complex playing and frequent and creative fills.
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