This is because many more buildings were constructed after 1840 and much larger numbers have survived, making greater selection necessary to identify the best examples.
Mae hyn am fod llawer mwy o adeiladau wedi’u hadeiladu ar ôl 1840 a bod llawer mwy ohonynt wedi goroesi, sy’n golygu bod angen bod yn fwy detholus er mwyn nodi’r enghreifftiau gorau.