Burn, Baby, Burn

You have a box of matches and two lengths of rope. Each length of rope will burn from one end to the other in exactly one hour. However, the burn rate along the length of each rope is not guaranteed to be uniform (ie: you don't know if the rope is, as an example, 60cm and burns at 1minute/cm all along its length; it might be different rates at different places along its length).

Using the two lengths of rope and the matches, how do you measure exactly fifteen minutes?