Hello Home Depot Customer, this is Colin from Simpson Strong-Tie. It seems we look at the term nominal in different ways. Simpson refers to nominal lumber that is machined and surfaced four sides (S4S). So a nominal 6x6 post measures 5-1/2" x 5-1/2". The ABU66Z will fit this nominal 6x6 post. A rough sawn post that is not S4S typically measures a net 6" x 6" and an ABU66RZ would fit this post. I am sorry for any confusion this may have caused. If you have any further questions please call us at 800-999-5099. We look forward to hearing from you.