The international evidence suggests consumption/sales taxes are complex within highly-integrated economic entities, such as that between border areas of Wales and England.
Mae’r dystiolaeth ryngwladol yn awgrymu bod trethi ar ddefnydd/gwerthiannau yn gymhleth o fewn endidau economaidd hynod o integredig, megis yr un rhwng ardaloedd ffiniol Cymru a Lloegr.