Quote:
One purpose of the polarized 2-prong plugs is to allow devices that would otherwise have required a heavier 3-prong cord to use "double-insulated equivalency" construction along with the 2-prong plug.

I don't think that answers the question of why the 2-prong plug must be polarised. In the UK all items with 2-conductor leads must be double-insulated, but many such items (e.g. laptop PSUs) have non-polarised "figure-of-8" connectors (which I've just found out Wikipedia calls C7, having no evidence of their being called "Telefunken" as I had thought).

Peter