A sonnet has 14 lines, among other conventions. Here’s one from Edna St. Vincent Millay.
I Shall Forget You Presently, My Dear
I shall forget you presently, my dear,
So make the most of this, your little day,
Your little month, your little half a year,
Ere I forget, or die, or move away,
And we are done forever; by and by
I shall forget you, as I said, but now,
If you entreat me with your loveliest lie
I will protest you with my favorite vow.
I would indeed that love were longer-lived,
And oaths were not so brittle as they are,
But so it is, and nature has contrived
To struggle on without a break thus far,—
Whether or not we find what we are seeking
Is idle, biologically speaking.