In a fraction, the denominator must be:

In a fraction, the denominator must be non-zero to ensure that the fraction is defined. A fraction represents a division of the numerator by the denominator; if the denominator were zero, it would mean dividing a number by zero, which is mathematically undefined. Thus, for any fraction to be valid and to hold a meaningful value in mathematics, the denominator has to be a non-zero number.

The relationship between the numerator and denominator being greater or less than each other does not determine the validity of the fraction, as fractions can be proper (numerator less than denominator), improper (numerator greater than denominator), or even equal (where the fraction equals one). Therefore, focusing on the requirement of the denominator being non-zero is key for understanding the foundational rules of fractions.