The National Hot Rod Association (NHRA) oversees most of drag racing events in North America. The next largest organization, Feld Entertainment's International Hot Rod Association (IHRA), is about one-third the size of NHRA. Nearly all drag strips are associated with one sanctioning body or the other. The NHRA is more popular with large, 1/4-mile, nationally recognized, tracks., The IHRA is a favorite of smaller 1/8-mile local tracks (and offers selected races on their national tour under the 1/8-mile format). One reason for this is the IHRA is less restrictive in its rules, such as rules on nitrous oxide (legal in Pro Modified) and oversized engines.