In American football, the 3–4 defense is a common defensive alignment consisting of three down linemen and four linebackers. It is a called a "base defense" because it is the default defensive alignment used on "base downs" (1st and 2nd downs). However, defenses will readily switch to other defensive alignments (such as a nickel defense or a dime defense) as circumstances change. Alternatively, some defenses use a 4–3 defense.