Trousers are plural for the same reason scissors are - their double (and probably duplicitous) nature.

Pearse was shot on May 3, not May 4.

This sent me looking for the Proclaimers and Letter from America, with its plangent list of the highland homelands emptying of emigrants, and its video image of the Bible in Gaedhlig, the bottle of whisky and the fiddle in the emigrant's bag fingered over by the Customs man.