Motherboards can come with either a 4 pin 12 volt connector or an 8 pin 12 volt connector. Many power supplies come with a 4+4 pin 12 volt cable which is compatible with both 4 and 8 pin motherboards. A 4+4 power cable has two separate 4 pin pieces. If you plug the two pieces of a 4+4 power cable together then you have a 8 pin power cable which can be plugged into an 8 pin 12 volt connector. If you leave the two pieces separate then you can plug one of the 4 pin pieces into a 4 pin 12 volt connector and leave the other 4 pin piece unplugged.
If you look carefully at the image above then you can see the polarization of the pins which prevents you from plugging the cable in improperly. Some of the pins are square and some of them have rounded off corners. The motherboard connectors have matching square and rounded off corners to prevent the cable from being plugged in the wrong way. But if you look really carefully at the right half of this particular cable and then look at the 8 pin 12 volt cable pictured above you'll notice that they don't match. A regular 8 pin cable has four square pins and four rounded ones but the 4+4 cable shown above has two square pins and 6 rounded ones. The left half of the 4+4 matches the left half of an 8 pin cable but the right half is different. Hmmmm... And this isn't some bizarre cable either. I've seen plenty of 4+4s which look like this one. And then there are other 4+4 cables which look just like an 8 pin cable split in two (which makes sense). Since rounded pins fit into square holes in motherboard connectors, this particular cable will fit just fine into an 8 pin 12 volt motherboard connector. But both halves of this 4+4 will fit into a 4 pin 12 volt motherboard connector. You're supposed to use the left half of the cable shown above when plugging it into a 4 pin motherboard connector but the right half will also fit. As it happens, either half will work fine in a 4 pin motherboard because both halves of the 4+4 just provide 12 volts. The pinouts are the same for both halves so either one will work. I'm not sure why they make cables like this one because you'd figure a 4+4 cable would just be an 8 pin cable which splits in two. And you only need one half of a 4+4 cable to plug into a 4 pin motherboard. The other half is unused. But the kind of 4+4 cable shown above is pretty common so don't let it throw you.
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