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A common factor is a factor that two or more numbers have in common or share.

**Example #1**

The factors of 6 are 1, 2, 3, and 6

The factors of 4 are 1, 2, and 4

A common factor of 4 and 6 is 2.

However, notice that 1 is also a common factor.

**Example #2**

The factors of 6 are 1, 2, 3, and 6

The factors of 4 are 1, 2, and 4

The factors of 15 are 1, 3, 5, and 15

The only common factor of 4, 6, and 15 is 1.

1 is always a common factor of two or more numbers because any number is divisible by 1.

The common factor is also called common divisor.