After you have written a verse and a chorus for your song, sometimes there will be a need for a transitional section between them. This could be due to the relationship between the lyric content of the two sections requiring some explanation, or it could be because the music from the verse needs to build energy into the chorus. Those are the two main purposes for adding a prechorus. The term prechorus will sometimes go by various other names such as the climb, or the transitional bridge. Not every verse-chorus song needs a prechorus, but if you feel that yours does, here are some tips on constructing a really good one.
Keep your prechorus short. Having a prechorus that is too long is the number one mistake that new songwriters make when constructing this type of section. Remember, a prechorus is a transitional section between two primary sections. About half the length of your verse or chorus is usually adequate, so if those two sections are about eight measures long each, then a four-measure prechorus would be a good length.
However, an asymmetrical section, one that has an odd number of measures, often works even better here for the overall structure of your prechorus section. If you construct your prechorus with three or five measures, instead of four, it has an off balancing effect that will push the music ahead into the next section, where the asymmetry will usually be brought into balance by a section with an even number of measures.
Here are some tips for the rhythm of your prechorus melodies. Shorter phrases in the prechorus, and/or phrases that enter more quickly, can build momentum into the next section. Additionally, employing faster rhythms, such as sixteenth notes, within your phrases can help build energy.
Here is the most commonly effective technique for the pitch content of your prechorus melodies. Gradually raising the pitch throughout the prechorus can be very effective at building energy into the chorus. This technique can also function as a connecting transition if the chorus is set at a higher pitch.
As for the lyric content of the prechorus, use it to explain the relationship between the verse and the chorus. Sometimes just a few short phrases will make all the difference in the world between your song making sense and completely losing your listeners.
Another important concept to keep in mind when it comes to the prechorus is contrast. Anything you can do musically to contrast this section with the sections that it is connecting will usually add helpful musical variety to the whole structure, so long as the contrast is not too extreme like changing the key or the time signature. Try using different chords, varying the pace at which the chords change, starting your phrases on different beats than that of the verse or chorus, and starting their melodies on different pitches. There are, of course many other ways to contrast sections, but these are the ones that I typically look for first.
So there you have it, the quick start guide to writing a great prechorus!