The sonnets are almost all constructed from three quatrains, which are four-line stanzas, and a final couplet composed in iambic pentameter. This is also the meter used extensively in Shakespeare's plays.
The rhyme scheme is abab cdcd efef gg. Sonnets using this scheme are known as Shakespearean sonnets. Often, the beginning of the third quatrain marks the volta ("turn"), or the line in which the mood of the poem shifts, and the poet expresses a revelation or epiphany.
There are a few exceptions: Sonnets 99, 126, and 145. Number 99 has fifteen lines. Number 126 consists of six couplets, and two blank lines marked with italic brackets; 145 is in iambic tetrameters, not pentameters. There is one other variation on the standard structure, found for example in sonnet 29. The normal rhyme scheme is changed by repeating the b of quatrain one in quatrain three, where the f should be.