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A choc block connector. When using stranded wire, first twist the strands together (blue wire) then fold them over (red wire).

These come in strips which can be easily cut according to the number of wires you need to connect. They are useful for connecting low voltage wires but offer no form of cable clamp so should only be used for mains if you can ensure that the cables cannot be strained. They contain a hole between each pair of connectors, which you can use to screw them down to a firm base.