Shakespeare's sonnet "Sonnet 138" Shakespearean sonnets are subject to a complex and exacting set of formal restrictions. They are 14 lines long and composed in a stanza. They are also composed in iambic pentameter and follow a certain rhyme pattern. The first line ends with an unstressed syllable followed by a stressed one, the second line ends with a stressed syllable followed by an unstressed one, and so forth. Finally, each quatrain (the four lines of a sonnet) ends with an incomplete enjambment, meaning that the final line ends with a partial word or phrase rather than a complete one. These elements together form a rigid structure that ensures that every sonnet contains the same number of words, has the same metrical rhythm, and uses the same rhyming patterns as any other. This is why it is easy to assign sonnets to their authors based on stylistic similarities alone. For example, many scholars believe that Shakespeare wrote some of his own poems because they show such clear signs of revision; others poems, including "Sonnet 138", seem to have been quickly written as responses to events of the time. In addition to these common features, most sonnets also share a central theme that focuses on love. This theme is expressed in different ways throughout the poem but can be found at its heart in almost all cases.