An arithmetic sequence is a sequence that has a constant difference between consecutive terms — that is, the difference between the first and the second term is the same as the difference between the second and the third term, and so on. This difference is called the common difference and is usually denoted with d. For example, consider the sequence of all even positive integers
For this sequence, the common difference is d=2. It is important to note that the common difference can take on negative values as well. Consider the following arithmetic sequence where the values decrease.
This is an arithmetic sequence with a common difference of -3.