emonstrate that frequency and phase are also dependent on each other, and that once phase alignment has occurred, maintaining a very close frequency alignment will keep the phase also closely aligned. This is a topic that will be revisited in the discussion about maintaining good phase alignment during holdover (a good place to start is the section "Holdover Performance" in Chapter 5). As shown in Figure 3-3, to determine phase alignment, an arbitrary repeatable point must be selec- ted to measure the phase or as a point to determine the phase from an oscillator's frequency signal. This point is known as the significant instant and usually lies somewhere on the rising or leading edge of a wave or pulse-the important factor is that it must be a repeatable point. Specific ex- amples could include the instant at which a signal crosses the zero baseline or reaches some value, such as the midpoint of the leading edge. Many early standards documents relating to distribution of timing signals through the physical layer define many timing concepts in terms of significant instants. But the concept is not difficult; it is simply the exact point of time when the receiver determines that the signal has "arrived" or has been "detected." An analogy might be the postmarking of a letter sent via the post office; the signi- ficant instant of posting a letter is that instant when the mail sorter cancels the stamp. Figure 3-4 shows examples of significant instants from a pair of ideal signal types. Significant instants Figure 3-4 Significant Instants in Sine and Square Wave When two signals have their significant instants occur at the same time, they are phase aligned. Note that the square wave (which might have a maximum voltage of 5 V) can never go from -5 V to +5 V instantly, as the electronics to do that do not exist. There will always be a tiny time window for the signal to lose the negative voltage and acquire the new positive voltage. The times taken for the signal to rise from the low level to the high level and back again 