The "standard" definition of a sonnet is fourteen lines of iambic pentameter, but such was not always the case. The most frequent departures from the standard, especially in the very early Italian sonnets involves a number of lines other than fourteen, or, especially in sonnets after the Elizabethan era, all sorts of metrical differences. The sonnet was an Italian poetic form that entered English literature by way of two sixteenth century English poets, Thomas Wyatt and Sir Henry Surrey. The two translated the sonnets of the medieval Italian poet Petrarch, their sonnets were printed in Tottle's Miscellany, and from there, the form became increasingly popular with court poets and musicians alike.

There are three principal sonnet styles or forms, with a number of minor variations and hybrid forms.

The Italian or Petrarchan sonnet is divided into two parts, in formal analysis: the octave, the first eight lines and the sestet, the next six. The octave has the rhyme scheme abbaabba, while the sestet usually has the rhyme scheme of cdecde or a variation, like cdcdcd or cdedce. The structure of the Petrarchan sonnet reinforces the sense and meaning of the sonnet, particularly in terms of the turn, or volta. The "turn" refers to the change in tone or subject presented in the ninth line, often indicated by beginning with a word like "then," or "but," a word that signifies a change or even a flat contradiction of what has been said in the previous eight lines.

The English or Shakespearean sonnet, associated with Shakespeare because he used the form with extraordinary skill (but not because it was invented by him), divides the fourteen lines into three quatrains (three groups of four lines) and a rhyming couplet. The rhyme scheme is almost invariably abab cdcd efef gg. The couplet usually functions much like the volta in the Petrarchan sonnet in that it presents an alternative, or a commentary or a resolution (or a contradiction) to the problem or situation presented in the previous twelve lines.

The Spenserian sonnet, created by Edmund Spenser for his sonnet sequence, the collection of sonnets he wrote for his fiance, his Amoretti, uses three quatrains and a couplet, with a rhyme scheme of abab bcbc cdcd ee. The rhymes link to each other, each quatrain repeatting a single rhyme from the previous quatrain, and reinforcing the sense of the lines. The "linking" of rhymes is as typical for Spenser as the final couplet rhyming in "gg" is typical for Shakespeare.