Running two loops at once

Hi.

I have a problem where I need to run two index parameters at one time.

For example, I need p to go from 1:21, but I also need an index n to run from -10:10 for each value of p. So when p=1, n =-10 etc

How do I do this?

[KSSV]

What's the necessity to make a loop index negative?