leader. When rehearsing or performing, the center snare may "tap off" the ensemble, setting the tempo with a solo rhythm. Musically, the snare drum section has the most rudimentally challenging "book", or music. The instrument's role is that of the soprano line, typically carrying the melody or the main rhythm of the ensemble. Marching snare drums are deeper in size than snares normally used for orchestral or drum kit purposes. This gives the drum the big, full sound necessary for outdoor use. Standard sizes (listed as diameter x depth) are 13x11 and 14x12 inches. Smaller sizes such as 13x9 have become increasingly popular in recent times with the proliferation of indoor drum lines. The modern "high tension" snare was developed in response to the higher head tensions made possible with the development of Kevlar and other high strength fibers bonded into the drum head. High tension drums began and were perfected in the pipe band market and later moved into the marching band and drum corps areas. The bottom (or resonant) side of the drum has a tightly tuned head and synthetic gut or metal snare wires, which are often secured to the drum using a strainer to limit their movement and make the sound more staccato. For outdoor use, a projector or "scoop" - a piece of curved plastic - may be attached to the back of the bottom hoop to help project the sound forward to the audience. Snare drums used in pipe bands are similar in construction to standard marching snare drums, with two key differences. First, the drum has an additional set of snares, directly under the batter (top) head. Second, the snares under the bottom head are made of coiled steel wires, similar to a drum set (as opposed to the synthetic "gut" snares on a corps-style drum). These differences tend to give the pipe drums a "snappier" snare sound, emphasizing the higher frequencies of the drum. Recently, corps-style drums have been produced with steel wire snares underneath the batter head (while remaining the gut snares under the bottom head). These snares are able to be switched on and off separate from the bottom snares, which allows units to use the second snares as a specific effect or as a permanent modification to the sound of the drum. The head of the snare drum can also be varied to give the drum a different sound.