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As digital system speeds increase and their sizes diminish, it becomes increasingly important to understand the mechanism of signal crosstalk (noise) in interconnections between logic elements. The worst case is when two wires run parallel for a long distance. Past literature has been unsuccessful in explaining crosstalk between parallel wires above a ground plane, because it was assumed that only one signal propagation velocity was involved. This paper proves that a signal introduced at one end of a printed wire above a ground plane in the presence of a second parallel (passive) wire must break up into two signals traveling at different velocities. The serious crosstalk implications are examined. The new terms slow crosstalk (SX), fast crosstalk (FX) and differential crosstalk (DX) are defined.