Jack Baker wrote:it is not meant to keep the spacing of notes perfectly even all the time, just to do the best we can and keep the overall tempo of the musical piece from speeding or slowing. The aggregate timing is all that can be asked from a metronome.
Well put, Jack.
Two points that I might add to this discussion:
1. We (at least "i") often need to be reminded that barlines are a relatively "new" or "young" concept in the history of music. Furthermore, they really evolved as a way to better organize music notation, not as way to make music "feel better". When music was written without barlines, it was written as phrases. Although a repeated phrase, with or without variations, would be played across the same time span, barlines would not be defined by the player. The more barlines you leave out, the more interesting your phrasing will be. Of course, this requires an impeccable sense of time, (time, in this sense, is not to be confused with "defining the beat"). Chet was real, real good at this - quite rare for solo instrumentalists who are both trying define the "American Beat" idea while also trying to make the melody feel "barlineless". Of course, Frank Sinatra was the master of barlinelessness but, he had a an orchestra and a conductor to help him keep things together.
2. We can develop this skill by progressively counting less (meaning to focus on fewer beats) while we practice. For example, play a two or four bar eighth note lick and focus on getting every note timed perfectly, i.e. count "in eight". Continue playing but start to count "in four"(here you are only worrying that the down beats being perfectly placed), then "in two" (here it is just beats 1 and 3, the +2+, and the +4+ can waiver), then "in one" (Chet's playing often felt like he was "thinking in one"). Then, just start worrying about every other beat one (two bar phrases are far and away the most predominant in American Music, and this is how it feels that Chet is thinking when he played in Cut Time, which was pretty often). When you start being able to focus just on whether or not every four bars you play are taking the same amount of time, you are in Sinatra territory, especially his early, crooning period.
This exercise can be a little frustrating and it can feel more than a little "scary" to trust that your inner clock is, in fact, capable of feeling large time spans to a remarkably accurate degree (and that so is your audience!), but, it is the time keepers master skill to feel all sorts of different time divisions, not just "define the beat".
Randy