The concepts applied to standard arithmetic are similar to those in more general algebraic structures, such as rings and fields. In a field, every nonzero element is invertible under multiplication; as above, division poses problems only when attempting to divide by zero. This is likewise true in a skew field (which for this reason is called a division ring). However, in other rings, division by nonzero elements may also pose problems. For example, the ring Z/6Z of integers mod 6. The meaning of the expression formula_41 should be the solution "x" of the equation formula_42. But in the ring Z/6Z, 2 is a zero divisor. This equation has two distinct solutions, and , so the expression formula_41 is undefined.