s happen at the same instant or point of time. One topic covered then was the music analogy where a metronome or an orchestra conductor is used to keep musical time. In fact, they have two purposes: one is a frequency task, to make sure music is played at the correct rate or tempo, expressed in beats per minute, and the second is to provide a regular "tick" that might signal the start of a new chord in a bar of music, or a wave of the baton to start a new movement. this analogy, these mechanisms are aligning every participant in the performance to a single agreed source of phase. Imagine an orchestra trying to play a piece of music with two different conductors doing their own thing! Frequency or tempo is a characteristic that can be observed and measured, as you can quickly de- termine if the musician is playing a piece too fast or too slow by independent observation and com- paring the rate at which the music is played to the tempo written in the music score. It might take a reference frequency source to determine if something is happening too slowly or too quickly, but it can be independently determined by checking the rate against a time source. Another example could be a nurse taking the pulse of a patient by checking the frequency of heartbeat against a wristwatch. Phase, on the other hand, cannot stand alone; it is always relative to a reference clock or a reference timescale. This means that phase alignment is based upon following the lead given by the source of the phase, and you cannot measure it as a standalone metric, only relative to the source. Continuing with the analogy, the phase signaled by the conductor is only relevant to the orchestra playing, and another orchestra playing elsewhere has their own phase. The important factor for phase is to measure how closely aligned the local phase is to the phase from the source of phase or some independent reference. So, you could say that machine A has a +100 ns phase offset to machine B, or machine B has a -250-ns phase offset compared to Coordin- ated U