Starting with the shape

I've started reading Hilary Menos's début collection Berg and the first line of the first poem struck me.

I want to write you a small square poem

I think this captures something fundamental about artistic creation ... the initial impetus for a new poem or a new piece of music might be simply some vague notion of the form or shape you want to create ... a sonnet, an unrhymed epigram, a piano sonata, a waltz, or an étude.