The main difference between Shakespearean sonnets and Petrarchan sonnets, apart from their origins, is the rhyme scheme and how the 14 lines of the poems are grouped.  Shakespearean sonnets use quatrains with the following structure: ABAB CDCD EFEF GG. Petrarchan sonnets instead combine an octave (eight lines) with a sestet (six lines) with a different rhyme scheme: ABBA ABBA CDE CDE.