is the emphasis on the first second or last bar?

Answer 1

As with most musical questions, it is confusing and it depends - it depends on the piece and it's rhythmic and formal structure. However, most popular music and jazz uses a 3/4 or 4/4 feel in a 12, 16, 24 or 32 bar form, so let's look at those.

In the same way that a piece may have sections: Verse, Chorus, Verse, Chorus, Solo, Chorus, each one of those sections can be broken down into smaller parts as well. Most pieces in these styles can be broken down into 4 bar sections (sometimes 8) that repeat, either the same or changed. A 4 bar phrase, and the emphasis of its bars is very similar to how the pulse works in a bar of 4/4: strong, weak, medium, weak. Bar one of a 4 bar phrase is the strongest and the third bar is the next strongest. The second and fourth bars are seen as weaker. The emphasis (longer, louder, more important notes) usually happens in the first bar, however, good composers and players will frequently "turn the beat around" by doing the opposite - which can be very effective, especially when it eventually resolves to "back to normal".

Now, there are other considerations as well, like flow of the phrase. The 4 bar phrase can also be looked at as a first half and a second half. Phrases that flow well usually have more movement in the second half of the phrase, and less in the first half with longer notes in the stronger bars. The thing that makes it confusing for writers is that this also happens at the two bar and one bar level - all at the same time (analogous to how a string vibrates at multiple pitches at the same time). Of course the flip-side of all this is that by shifting just one or two accents you can create a completely different emotional feel.