Which namespace for DocumentFormat

I'm about to give up with Word automation and am going to try using Open XML.

It seems I need references to:

uysing System.IO
using System.IO.Packaging;
using DocumentFormat.OpenXml.Packaging

After a bit of ferreting around I found you need to add a reference to Windows Base to get System.IO.Packaging to appear.

What do you need to reference to get to use DocumentFormat.OpenXml.Packaging?

Thanks for any help.