A very useful plumbing part to connect different sized female threaded fittings is a reducer nipple (sometimes also call a reducing nipple).
We've added 2 sizes - one that reduces from 3/4 to 1/2 and the other that reduces from 1/2 down to 3/8 which is a common European size.
We've gone for the premium end of the market with ours made from chrome plated solid brass.
They can be found here: Chrome Reducing Nipple