All sonnets follow a rather rigid rhyme scheme. In the Italian sonnet, the first eight lines are written in a rhyme scheme of "abba abba." This means that the first, fourth, fifth and eighth lines rhyme with each other, as do the second, third, sixth and seventh. The final six lines in the Italian sonnet use some combination of two lines of c, two of d and two of e. This can take the form of "cde cde," "cdc ede," "cde ced" or any other variety.
Like the English sonnet, Italian sonnets make use of iambic pentameter. This means each line of the sonnet contains five metrical units, each consisting of an unstressed syllable followed by a stressed syllable. For example, the first line of a famous sonnet by John Milton reads, "When I consider how my light is spent" and follows this meter exactly. It should be noted that sonnets often deviate from this meter, but usually to draw attention to some thematic point for emphasis.
The first eight lines of Italian sonnets are called an octave, which is made up of two quatrains of four lines each. Typically, the octave presents some kind of problem that speaker is trying to figure out. The octave is, in a sense, a proposition that must be resolved somehow. The final six lines of the sonnet are the sestet, and work toward finding a solution to the problem raised in the octave. The sestet typically begins with a volta, which is a change in perspective that might begin a with a transitional word, such as, "but."
The major structural differences between Italian and English sonnets are in relation to the rhyme scheme and stanzaic pattern. English sonnets have a rhyme scheme of "abab cdcd efef gg." Importantly, the English sonnet ends with a rhyming couplet, which typically produces some sort of structural closure, often mimicked by closure in terms of the poem's narrative content. Because Italian sonnets usually don't end with a couplet, they lack the formal and thematic closure that an English sonnet provides.