They are devoted to the main idea of the poem, with the poet talking of his mistress in less than complimentary terms. The first twelve lines rhyme in alternating pairs. Sonnet 130 follows the rhyme scheme ABABCDCDEFEFGG. Sometimes this pattern changes, which can tell you something about the importance of the line.
In iambic pentameter the rhythm goes ‘unstressed, stressed’. In each foot there is one stressed syllable. Pentameter means that each line is divided up into five feet. This is a name for a certain pattern of beats called ‘feet’. Shakespearean sonnets are written in iambic pentameter.
Shakespeare didn’t invent the form, but he did help popularise it. A sonnet is a poem of 14 lines that follows a strict rhyming pattern.
