The dimensions that can be formed from a given collection of basic physical dimensions, such as T, L, and M, form an abelian group : The identity is written as 1; L = 1, and the inverse of L is 1/L or L. L raised to any integer power p is a member of the group, having an inverse of L or 1/L. The operation of the group is multiplication, having the usual rules for handling exponents (L × L = L). Physically, 1/L can be interpreted as reciprocal length, and 1/T as reciprocal time (see reciprocal second).