Musical form, and the music heard as a whole
How sections are organised and repeated; nearly every song fits into a few letters, and knowing the form is what lets you play a piece by ear after two listens.
A phrase ends in a cadence. Several phrases make a section. Several sections make the piece. Form is how the sections are organised and repeated, and it is the level at which music is heard as a whole — the level at which you know, without counting bars, that "the chorus is coming". Nearly every song fits into a few letters.
Letters for sections
The convention is to give each different section a letter and to repeat the letter when the section comes back. The first section is A; a different section is B; if the first returns, it is A again. The form is the sequence of letters.
A A B A is the form of the classic American song and of much bossa nova: a section that presents itself, repeats, contrasts, and returns. Thirty-two bars, eight per section, and a whole repertoire fits inside it.
Verse and chorus, or A B A B, is the form of popular radio song: the verse tells, the chorus repeats, and the alternation is the music. A bridge, C, usually appears once before the last chorus.
A B, two sections with no return, is the form of the classical choro with three parts and of the waltz, in which each part comes back by internal repetition.
The cadence marks the phrase
How do you know where a section ends? By the cadences. A phrase ends in a cadence; when an authentic cadence closes and the music starts again on something already heard, a section has ended. Form is read from the bottom up: cadences mark the phrases, phrases mark the sections, and the sections draw the form.
Two four-chord phrases, the first ending on a half cadence, the dominant, and the second the same. If the second ended on the tonic, the two together would be a question-and-answer section, eight bars, an A.
Recognising form by ear
Knowing the form is what lets you play a song after hearing it twice. You do not memorise every chord: you memorise that there is an eight-bar A ending on the dominant, an A ending on the tonic, a B that goes to the fourth degree, and the final A. Four pieces instead of thirty-two bars, and two of them identical.
The exercise is to listen to a song counting sections instead of chords: where it begins, where it repeats, where it changes, where it returns. On the second listen, write the letters. On the third, the chords of each section, which are almost always few.
The bridge and the coda
Two sections have names of their own. The bridge is the contrasting section that appears once, generally before the last return, and usually goes somewhere else harmonically — the fourth degree, the relative, a neighbouring key — before preparing the return with a dominant. The coda is what is added after the last return in order to finish: a repeat of the final phrase, a stretched cadence, the turnaround that finally does not turn around.
A short bridge going to the fourth degree, passing through a secondary dominant and ending on the dominant, ready for the A to return.
What to take from here
Form is the organisation of the sections, written in letters: A A B A, verse and chorus, A B. Cadences mark the phrases, phrases mark the sections, and form is read from the bottom up. Recognising it is what turns thirty-two bars into four pieces, and it is the skill that lets you play by ear after two listens.