Both marks on this page change when a chord happens without touching what the chord is, which is exactly why new readers walk straight past them. The harmony looks familiar, the page looks easy, and then the band arrives somewhere half a beat before you do.
The caret is a push: the chord lands on the eighth note before the downbeat, and the downbeat itself is left alone. Written in the bar it belongs to, played in the bar before it — that sentence is the whole difficulty. The diamond is a hold: hit it, let it ring, and stop counting until the next bar.
Below, the same eight bars in A and then in D — a fourth up, the single most common move a session makes to a written page. The middle column is what is actually printed, and it is the column that does not change.
| Bar | Written | In A | In D |
|---|---|---|---|
| 1 | 1 | A | D |
| 2 | <4 | D | G |
| 3 | 1 | A | D |
| 4 | <5 | E | A |
| 5 | 4 | D | G |
| 6 | 4 5sus | D Esus4 | G Asus4 |
| 7 | 57 | E7 | A7 |
| 8 | ◇1 | A | D |
Why these are on the page at all
Almost nothing about rhythm is written on a number chart. The groove is the record's business, the feel is the drummer's, and a chart that tried to specify either would take an hour to write and be unreadable at a count-off. So when a rhythmic mark does appear, it is there because the arrangement genuinely depends on it and the band would not have guessed.
A push is that kind of thing. If two players anticipate the chord and three do not, the result is not a subtle difference of interpretation — it is an audible collision on a downbeat. The mark exists to make five people wrong or right together.
The diamond in the last bar is doing the same job at the other end of the scale. Everything stops; the chord rings; the count picks up when it picks up. That is unwritable in any other way short of a conductor, and it is the mark most likely to be missed by somebody reading the page as a list of chords rather than as a piece of time.