I find myself, presently, saddled with the odd conundrum of the order in which to arrange the words “sombre”, “solemn” and “slow” in a line. “Slow” is easy, it is the rhyming element of the line and therefore must come last, but its companions are deliberately so similar in construction and pronunciation that they are interchangeable.

This is an annoyance to a pedant who wants a rationale for all things.

Of course, the purpose here is the similarity and, therefore, interchangeability—perhaps I should look to visual alliteration for the answer: “ol” in “solemn” appears reversed in “slow” and thus “Solemn, sombre and slow” is aesthetically most pleasing—but what a pity there is no linguistic rule (that I know) that specifically here applies!