Some combustion chamber designs, such as rotary motors, require that the spark plugs have the ground electrode placed to the side of the center electrode rather than below as on a traditional plug (see photos below.) This firing tip design tends to erode the tip of the ground electrode faster than a traditional plug, and as erosion at these points creates a larger gap between the center and ground electrodes, plug misfire will occur. Thus, by having more ground electrodes, you extend plug life. It is important to note that multi-ground does not mean multi-spark - electricity follows the path of least resistance, there will still only be one spark at a time. Therefore a multi-ground plug will not perform any better and may actually perform worse than a traditional plug, unless the engine is designed for a multiple ground plug.
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