Concurrent requests to a shared variable by many processors on a shared-memory machine can create contention serious enough to stall large machines. This idea has been formalized in the hot spot traffic model in which a fixed fraction of memory requests is for a single shared variable. Combining, in which several requests for the same variable can be combined into a single request, has been suggested as an effective method of alleviating this contention. The NYU Ultracomputer and the IBM RP3 machine use pairwise combining, in which only two requests for the same variable can be combined at a switch. A study is made of the effectiveness of combining. It turns out that pairwise combining cannot handle hot spots if the machine size is large enough. Suggestion are put forth to overcome this weakness.