This is a simple question but I don't have a rulebook at hand.

Are the base lines or foul lines in fair or foul territory? That is, do you use the inside (toward the playing field) or the outside of the lines to determine whether a ball is in fair our foul territory when it touche the line?

You're right, it is simple :)

The base lines...the entire line...are in fair territory. So are the bases, including home plate.

If it touche the line, it's fair.