Problems locating the assembly for Microsoft.Office.Core.DocumentProperties

I am playing around with Word 2007 in Visual Studio 2008.
I was trying to set document properties using code in the form:

( (Microsoft.Office.Core.DocumentProperties) mWordDocument.BuiltInDocumentProperties )[ Word.WdBuiltInProperty.wdPropertyCompany ].Value = "My Company Name";

I had referenced C:\Program Files\Microsoft Visual Studio 9.0\Visual Studio Tools for Office\PIA\Office12\Microsoft.Office.Interop.Word.dll which was available in the GAC but I could not find the assembly which contained the definitions for Microsoft.Office.Core.DocumentProperties.

I found it at C:\Program Files\Microsoft Visual Studio 9.0\Visual Studio Tools for Office\PIA\Office12\Office.dll

Hope this helps another frustrated searcher.