What is the plural form of 'trade union'?

Answer 1

The correct plural of trade union, is 'trade unions' (meaning more than one trade union).

The form 'trades union' is historical, going back to the early unions which would be formed with workers from more than one trade (hence 'trades'). This then later changed as unions became more specific to particular trades to simply 'trade union'. It could be argued that this has swung the other way these days with unions taking people from many different 'trades', however the form 'trade union' has stuck.

There are exceptions though, so the TUC (Trades Union Congress), is written that way simply because they wanted to keep to the older, perhaps traditional, meaning.