A load of yobs from PenrithPedanticus writes: It has to be pointed out that the "th" in "with" is voiced, making strict rhyming impossible. Using Welsh is not possible because the stress would be on the wrong syllable. The only solution to this impasse is to assume marked Caledonian chararcteristics on the part of Superman
(KagShu) I'd say the difference is like the difference between the th's in thing and there, this, that etc. I don't think there's any difference between American and British English on this point.
[Superman] You can hope, and you can stack the deck with a difficult rhyme in the first line, but in the end you get no more say in the finished product than if you'd played a move that offered wide-open possibilities to everyone else.
(Raak) I have here a copy of Prout's "Counterpoint, Strict and Free". It was bought by my Dad in Tunbridge Wells and he has put the date (Jan 18 1941) and his name and address on the flyleaf. The address is in Crowborough, where I was born, but I've no idea if it was where I lived for the first two years or whether it was in one or both of two other addresses in said town. Should've asked - bit late now. Prout's volume you could call "severe".
My father, a violinist, had a copy, and my brother, a cellist, no doubt looked into it during his musical education. I was always puzzled by where all of these rules came from and what their purpose was, but I could never get an explanation.