Denominator (Number on bottom)

The denominator is the nonzero lower entry $b$ in a fraction $a/b$. It specifies the division being performed; with $b>0$, the unit fraction $1/b$ is the size of one part. Thus $3/5$ and $3/10$ have the same numerator but different values.

An integer fraction can always be written with positive denominator. Dividing both entries by their greatest common divisor gives a reduced fraction, so $6/10$ and $3/5$ represent the same rational number despite different denominators.

A zero denominator is excluded from ordinary rational numbers. The symbol $1/0$ used at the boundary of the Stern-Brocot tree is a formal boundary symbol, not an additional rational number produced by division by zero.