The division into Octave and Sestet mainly applies to Petrarchan sonnets. In a Shakespearean sonnet the sestet is less important (sometimes not important at all). The Petra
…chan sonnet works by taking two views of its topic. The main idea comes in the first eight lines (the Octave) then a slightly different point of view is presented in the closing six (the Sestet). The change of viewpoint is often called the Volta (and is really the defining feature of a Petrarchan sonnet). Poems can tend to be a bit one-sided, with the poet presenting his point of view - and the reader has to agree with this, or else find a different poem. (This is a huge problem with Italian Canzone or the early Renaissance - the sort of poem Petrarch was trying to react against; it is also a huge issue with most contemporary poetry). But because the Petrarchan sonnet has a Volta, it naturally presents two points of view. This means that instead of the reader being given the poet's opinion - on a take-it-or-leave-it basis - the poem itself invites the reader to think for herself (by weighing up the differences between the two points of view). This second-opinion aspect of the Petrarchan sonnet makes it enormously more difficult to write successfully, and quite a bit more challenging to read - compared with a standard 'Oh What a Beautiful Morning' type poem. But quite often a poem which makes you think for yourself is one which will stay with you longer This inherent dynamic quality of the sonnet is a main reason why the form has lasted so long, and has produced so many truly great poems. (MORE)