The arrow normally represents the #1 spot or lowest number pin. This is usually the ground. Most of the time this does not matter for these plugs. It is just an open/close switch or a speakers. The +/- does not matter.
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Yes, the plugs that go onto the pins on the motherboard. They are coloured coded, but is the triangle or 'arrow' the plus or the minus wire, reason I ask is because on some of the plugs the arrow is where the coloured wire is terminated but it also appears where the white wire is terminated ? I guess overall the coloured wire is meant to be the plus wire and the white wire is the minus wire ?