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A solder join - wires spliced together (preferred method).

A solder join - wires twisted together (not so good).

Twisting the wires together and soldering is often the best method, but there are two ways of doing this. If possible, you should twist the wires in-line before soldering as this makes a stronger (and neater) join than twisting the ends together. This requires a greater length to be stripped of insulation and can be tricky if the wires differ in diameter, or if one is stranded and the other solid.