Song structure

Song structure is the arrangement of a song, and is a part of the songwriting process. It is typically sectional, which uses repeating forms in songs. Common piece-level musical forms for vocal music include bar form, 32-bar form, verse–chorus form, ternary form, strophic form, and the 12-bar blues. Popular music songs traditionally use the same music for each verse or stanza of lyrics. Pop and traditional forms can be used even with songs that have structural differences in melodies. The most common format in modern popular music is introduction, verse, pre-chorus, chorus, verse, pre-chorus, chorus, bridge, and chorus, with an optional outro. In rock music styles, notably heavy metal music, there are usually one or more guitar solos in the song, often found after the middle chorus part. In pop music, there may be a guitar solo, or a solo performed with another instrument such as a synthesizer or a saxophone.
The foundation of popular music is the "verse" and "chorus" structure. Some writers use a simple "verse, hook, verse, hook, bridge, hook" method. Pop and rock songs nearly always have both a verse and a chorus. The primary difference between the two is that when the music of the verse returns, it is almost always given a new set of lyrics, whereas the chorus usually retains the same set of lyrics every time its music appears." Both are essential elements, with the verse usually played first. Each verse usually employs the same melody, while the lyrics usually change for each verse. The chorus usually consists of a melodic and lyrical phrase that repeats. Pop songs may have an introduction and coda, but these elements are not essential to the identity of most songs. Pop songs often connect the verse and chorus via a pre-chorus, with a bridge section usually appearing after the second chorus.
The verse, chorus and pre-chorus are usually repeated throughout a song, while the intro, bridge, and coda are usually only used once. Sometimes a post-chorus will be present on a song. Some pop songs may have a solo section, particularly in rock or blues-influenced pop. During the solo section, one or more instruments play a melodic line which may be the melody used by the singer, or, in blues or jazz improvised.

Verse–chorus form

consists of two main sections – a verse and chorus – that often contrast melodically, rhythmically, harmonically and dynamically. Songs in verse–chorus form may also include introductory, transitional and concluding sections.

Introduction

The introduction is a unique section that comes at the beginning of the piece. Generally speaking, an introduction contains just music and no words. It usually builds up suspense for the listener so when the downbeat drops in, it creates a pleasing sense of release. The intro also creates the atmosphere of the song. As such, the rhythm section typically plays in the "feel" of the song that follows. For example, for a blues shuffle, a band starts playing a shuffle rhythm. In some songs, the intro is one or more bars of the tonic chord. With songs, another role of the intro is to give the singer the key of the song. For this reason, even if an intro includes chords other than the tonic, it generally ends with a cadence, either on the tonic or dominant chord.
The introduction may also be based around the chords used in the verse, chorus, or bridge, or a stock "turnaround" progression may be played, such as the I–vi–ii–V progression. More rarely, the introduction may begin by suggesting or implying another key. For example, a song in C Major might begin with an introduction in G Major, which makes the listener think that the song will eventually be in G Major. A cliche used to indicate to the listener that this G Major section is in fact the dominant chord of another key area is to add the dominant seventh, which in this case would shift the harmony to a G⁷ chord. In some cases, an introduction contains only drums or percussion parts that set the rhythm and "groove" for the song. Alternately the introduction may consist of a solo section sung by the lead singer, or a riff played by an instrumentalist.
The most straightforward, and least risky way to write an introduction is to use a section from the song. This contains melodic themes from the song, chords from one of the song's sections, and the beat and style of the song. However, not all songs have an intro of this type. Some songs have an intro that does not use any of the material from the song that is to follow. With this type of intro, the goal is to create interest in the listener and make them unsure of what will happen. This type of intro could consist of a series of loud, accented chords, punctuated by cymbal, with a bassline beginning near the end, to act as a pitch reference point for the singer.

Verse

In popular music, a verse roughly corresponds to a poetic stanza because it consists of rhyming lyrics most often with an AABB or ABAB rhyme scheme. When two or more sections of the song have almost identical music but different lyrics, each section is considered one verse.
Musically, "the verse is to be understood as a unit that prolongs the tonic....The musical structure of the verse nearly always recurs at least once with a different set of lyrics." The tonic or "home key" chord of a song can be prolonged in a number of ways. Pop and rock songs often use chords closely related to the tonic, such as iii or vi, to prolong the tonic. In the key of C Major, the iii chord would be E Minor and the vi chord would be A Minor. These chords are considered closely related to the tonic because they share chord tones. For example, the chord E Minor includes the notes E and G, both of which are part of the C Major triad. Similarly, the chord A Minor includes the notes C and E, both part of the C Major triad.
Lyrically, "the verse contains the details of the song: the story, the events, images and emotions that the writer wishes to express....Each verse will have different lyrics from the others." "A verse exists primarily to support the chorus or refrain...both musically and lyrically." A verse of a song, is a repeated sung melody where the words change from use to use.

Pre-chorus

An optional section that may occur after the verse is the pre-chorus. Also known as a "build", "channel", or "transitional bridge", the pre-chorus functions to connect the verse to the chorus with intermediary material, typically using subdominant or similar transitional harmonies. "Often, a two-phrase verse containing basic chords is followed by a passage, often harmonically probing, that leads to the full chorus." Often, when verse and chorus use the same harmonic structure, the pre-chorus introduces a new harmonic pattern or harmony that prepares the verse chords to transition into the chorus.
For example, if a song is set in C Major, and the songwriter aims to get to a chorus that focuses on the dominant chord being tonicized, a chord progression could be used for the pre-chorus that gets the listener ready to hear the chorus' chord as an arrival key. One widely used way to accomplish this is to precede the G Major chord with its own ii–V⁷ chords. In the key given, ii of G Major would be an A minor chord. V⁷ of G Major would be D⁷. As such, with the example song, this could be done by having a pre-chorus that consists of one bar of A minor and one bar of D⁷. This would allow the listener to expect a resolution from ii–V to I, which in this case is the temporary tonic of G Major. The chord A minor would not be unusual to the listener, as it is a shared chord that exists in both G Major and C Major. A minor is the ii chord in G Major, and it is the vi chord in C Major. The chord that would alert the listener that a change was taking place is the D⁷ chord. There is no D⁷ chord in C Major. A listener experienced with popular and traditional music would hear this as a secondary dominant. Harmonic theorists and arrangers would call it V⁷/V or five of five, as the D⁷ chord is the dominant chord of G Major.

Chorus or refrain

The terms chorus and refrain are often used interchangeably, both referring to a recurring part of a song. When a distinction is made, the chorus is the part that contains the hook or the "main idea" of a song's lyrics and music, and there is rarely variation from one repetition of the chorus to the next. A refrain is a repetitive phrase or phrases that serve the function of a chorus lyrically, but are not in a separate section or long enough to be a chorus. For example, refrains are found in the Beatles' "She Loves You", AC/DC's "You Shook Me All Night Long", Simon & Garfunkel's "The Sound of Silence", and "Deck the Halls".
The chorus or refrain is the element of the song that repeats at least once both musically and lyrically. It is always of greater musical and emotional intensity than the verse. "The chorus, which gets its name from a usual thickening of texture from the addition of backing vocals, is always a discrete section that nearly always prolongs the tonic and carries an unvaried poetic text." In terms of narrative, the chorus conveys the main message or theme of the song. Normally the most memorable element of the song for listeners, the chorus usually contains the hook.

Post-chorus

An optional section that may occur after the chorus is the post-chorus. The term can be used generically for any section that comes after a chorus, but more often refers to a section that has similar character to the chorus, but is distinguishable in close analysis. The concept of a post-chorus has been particularly popularized and analyzed by music theorist Asaf Peres, who is followed in this section.
Characterizations of post-chorus vary, but are broadly classed into simply a second chorus or an extension of the chorus. Some restrict "post-chorus" to only cases where it is an extension of a chorus, and do not consider the second part of two-part choruses as being a "post"-chorus.
As with distinguishing the pre-chorus from a verse, it can be difficult to distinguish the post-chorus from the chorus. In some cases they appear separately – for example, the post-chorus only appears after the second and third chorus, but not the first – and thus are clearly distinguishable. In other cases they always appear together, and thus a "chorus + post-chorus" can be considered a subdivision of the overall chorus, rather than an independent section.
Characterization of a post-chorus varies, beyond "comes immediately after the chorus"; Peres characterizes it by two conditions: it maintains or increases sonic energy, otherwise it is a bridge or verse; and contains a melodic hook, otherwise it is a transition.
Detached post-choruses typically have distinct melody and lyrics from the chorus:

"Chandelier" : the chorus begins and ends with "I'm gonna swing from the chandelier / From the chandelier", while the post-chorus repeats instead "holding on", in "I'm holding on for dear life" and "I'm just holding on for tonight", and has a new melody, but the same chord progression as the chorus.
"The Boys Are Back in Town" : the chorus consists of the phrase "the boys are back in town", repeated in a call-and-response style, while the post-chorus features a prominent riff by two lead guitars.
"Smells Like Teen Spirit" : the chorus lasts from "With the lights out, it's less dangerous" to "A mosquito, my libido", while the post-chorus features a heavy riff with the vocals "hey, yay".

Lyrics of attached post-choruses typically repeat the hook/refrain from the chorus, with little additional content, often using vocables like "ah" or "oh". Examples include:

"Umbrella" : the chorus begins "When the sun shine, we shine together" and run through "You can stand under my umbrella / You can stand under my umbrella, ella, ella, eh, eh, eh", which is followed by three more repetitions of "Under my umbrella, ella, ella, eh, eh, eh", the last one adding another "eh, eh-eh". Here the division between chorus and post-chorus is blurred, as the "ella, ella" begins in the chorus, and was a play on the reverb effect.
"Shape of You" : the chorus runs "I'm in love with the shape of you... Every day discovering something brand new / I'm in love with your body", and the post-chorus repeats vocables and the hook "Oh—I—oh—I—oh—I—oh—I / I'm in love with your body", then repeats the end of the chorus, switching "your body" to "the shape of you": "Every day discovering something brand new / I'm in love with the shape of you"
"Girls Like You" : the chorus runs "'Cause girls like you... I need a girl like you, yeah, yeah... I need a girl like you, yeah, yeah", and the post-chorus repeats the hook with added "yeah"s: "Yeah, yeah, yeah, yeah, yeah, yeah / I need a girl like you, yeah, yeah / Yeah yeah yeah, yeah, yeah, yeah / I need a girl like you".

Hybrids are also common, where the post-chorus keeps the hook from the chorus, but introduces some additional content (hook or melody, like a detached postchorus.