The structures we hear in popular music have been developed and established as a result of many years of repetition. There's a logic behind the concepts of introductions, middle 8s, bridges and choruses that mirrors the ideas of narrative in literature. There's no reason to follow these rules if you don't want to, but they exist because people connect with them and can be a powerful tool if you choose to familiarize yourself with them.
Intros and Outros
Intros and outros often use the same chords and melodies as the verse or chorus parts, in order to create a sense of recognition and to tie a whole song together. A common technique is slowing the tempo down compared to the pace of the rest of a song. It creates a sense of gradually building toward the chorus from the very start of the song. Alternately, you can create some interesting dynamic effects by using a totally different melody for your intro, which will increase the impact by creating a sudden, jarring shift when the main melody arrives.
Pre-Chorus or Lift
This section, unsurprisingly, usually appears immediately before the chorus. It provides a transition between the verse and chorus. Often, it's used to create suspense or tension by dropping off the tempo of the verse down to a much slower pace, before bursting back into the chorus. As with the introduction, deliberately creating this contrast can increase the overall impact. Also, if your song features a very simple progression, where the verses and chorus are made from the same chords, a pre-chorus can break it up and help "announce" the transition.
A song generally only includes one of these, and as a rule, they usually appear before the final verse or chorus. Their role is similar to that of the pre-chorus, to break up the verse-chorus-verse-chorus repetition and build tension before a crescendo. Middle 8s often use a different progression than the rest of a song, sometimes in a completely different key. Provided you can transition into it from the previous section and have an "exit strategy" to get into the next one, its not necessary for it to bear any resemblance to the rest of the song.
The standard "intro-verse-chorus-verse" structure is one that appears countless times in popular music. Audiences identify with the narrative structure, and it has obviously proven itself successful. However, there's no reason you can't discard these ideas and go your own way. Genres as diverse as folk, jazz, blues, techno and hip-hop have all shown you can create compelling songs without things such as choruses or Middle 8s. Provided the transition of each section into the next creates the desired effect with the listener, there are no set rules.
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