Have a long hallway or a crevice you're looking to fill?
One recommendation is to compliment the centerpiece rugs of your floor space with "runners" or long rectangular rugs.
Several tribes including the Kashkai, the Yalameh, the Shiraz, and the Bakhtiari all make rugs called runners that are no more than 3 feet wide and could be anywhere up to 16 feet long.
The runners are generally designed to fill in all the corners of a room. As shown in the picture below with a Yalameh runner, the runner is laid on the head of another rug so that less of the floor space is left uncovered.
Runners are very expensive because they're difficult to make and very few tribes create them accurately. If the rug isn't created perfectly straight when it comes off the loom, it is warped and then it wears unevenly.