The Italian, or Petrarchan, Sonnet is written in iambic pentameter. The sonnet consists of fourteen lines, separated into an eight line stanza and a six line stanza. The first stanza (with eight lines) is called an octave and follows the following rhyme pattern:
a b b a a b b a.
The second stanza (consisting of six lines) is called a sestet and follows one of the following rhyme patterns:
c d c d c d c d e c d e c d e c e d c d c e d c
c d d c d c.
The final two lines cannot end in a couplet (given the couplet was never used in Italy or by Petrarch).
The change in both rhyme pattern and subject matter takes place by the creation of two distinct stanzas (the octave and the sestet). The change in rhyme and subject happen at the volta, the ninth line of the poem (the first line of the second stanza).
The Shakespearean Sonnet, or English Sonnet, is very different from the Petrarchan Sonnet. While the Shakespearean Sonnet consists of fourteen lines (like the Petrarchan Sonnet), the lines are divided into stanzas very differently.
This sonnet is composed using three quatrains (three stanzas consisting of four lines each) and a concluding couplet (a two line stanza). The rhyme scheme of this sonnet is alternating, throughout the quatrains, and ends in a rhyming couplet.
Therefore, the rhyme scheme of the Shakespearean Sonnet is as follows:
a b a b
c d c d
e f e f
Both the Petrarchan and Shakespearean Sonnets have a place where the subject changes, but in the Shakespearean Sonnet it is not called the volta, instead, it is called the turn. The turn takes place at the same point (line 9) as the Petrarchan Sonnet. Sometimes though, the turn may not happen until the couplet.
Here is a video further describing the differences:
thank you so much for great help..