Some MBs have one 20-pin jack; some have a 24-pinner, and some have a 20-pinner and a second 4-pinner. Most new PSUs now come with a 20+4 MB connector, in which case: With a MB having just one 20-pin jack, the smaller 4-pin section of the 20+4 is not used. The entire 20+4 is used with a 24-pin MB jack. The tricky part comes with a MB that has two power jacks -- a 20-pinner and a separate 4-pinner. In this case, the smaller section of the 20+4 is the polar opposite of the 4-pin jack and should not be used, even though it can be forced into the jack. Instead, find the PSU's other square 4-pinner that's properly polarized and fits the MB like a glove. I once forced the wrong plug into my MB with, let's just say, less than favorable results.