The common difference of an arithmetic sequence is the constant difference between consecutive terms.

In the sequence: 17, 19, 21, ...

To find the common difference ($d$):

$d = \text{Second term} - \text{First term} = 19 - 17 = 2$

Thus, the common difference is 2.

Would you like further explanation or details? Here are five related questions you might find interesting:

1. What is the general formula for the $n$-th term of an arithmetic sequence?
2. How can you find the sum of the first $n$ terms of an arithmetic sequence?
3. If the first term of an arithmetic sequence is 17 and the common difference is 2, what is the 10th term?
4. How do you distinguish between an arithmetic and a geometric sequence?
5. What happens to the common difference if you add a constant to all terms of the sequence?

Tip: In arithmetic sequences, the difference between any two consecutive terms is always constant.