The notation needs a little clarification. When only one wire is shown, such as the WR wire, it is either 0 or 1. A lower line represents 0 and the higher level represents 1, with a short diagonal line between any transitions to represent the fact that the values of wires do not change instantaneously. When a group of wires is shown, such as the RI wires (the three RAM Input wires of Fig. 5.10.1), either a single line is shown, or a "fat" line, with a half-diamond to represent the transitional phase. When the line is "fat," the group of wires has values that we care about, although some may be 0 and some may be 1. When the line is thin or single, the group of wires either has old or invalid data on it, or does not contain values that we care about.