The sonnet is conventionally identified by its length, and identification by length explains nothing about why the form has survived five centuries of use in several languages. Fourteen lines is an arbitrary quantity. What distinguishes the sonnet from any other short poem is not its extent but its articulation: the form incorporates a structural division, and the division is where the work is done.
In the Italian arrangement the division falls after the eighth line, separating a proposition of eight lines from a resolution of six. The English arrangement postpones it, accumulating three quatrains and concluding with a couplet that must accomplish in two lines what the Italian resolution has six in which to manage. The two produce characteristically different poems -- the Italian expansive in its second movement, the English epigrammatic -- and the difference is entirely a consequence of where the division is placed.
What the division permits is a reversal that the reader is prepared for and does not anticipate. The opening establishes a position with sufficient conviction that the reader accepts it; the turn then qualifies, contradicts or reinterprets it, and the qualification carries the force of a discovery because the reader has been allowed to settle. A poem of the same length without a turn is a description. A poem with a turn in an unexpected place is a sonnet that has been argued with, which is why departures from the conventional position are conspicuous and are usually meant to be.
An invented example will show the mechanism. A poem opens by setting out, over eight lines, the case for regarding a ruined mill as a straightforward loss: the roof gone, the floor open to the weather, the rooms that once held a working household now holding nothing at all. The reader is given no reason to resist the description, and by the eighth line he has agreed to it. The ninth line begins with a single word of qualification. What follows is the observation that the mill is now frequented by people who would never have entered it whole, and that its ruin is the condition of their being there. Although nothing in the first eight lines is withdrawn, the reader is handed a second thing to hold beside the first. The second arrives with the force of a discovery precisely because he was allowed to settle into the first.
This accounts for a fact about imitation that length alone does not. Poets who abandon rhyme, or metre, or the fourteen lines themselves, are still recognisably writing sonnets provided the articulation survives. Poets who preserve every formal requirement and write continuously are not, and readers register the absence immediately without necessarily being able to name it. The recognisable element is the one that was never counted.
