Simpson Strong Tie offers products to accommodate both nominal and rough sawn cut lumber. When it comes to lumber sizing, most people are familiar with the sizes 2x4 and 2x6. However, if you go to a lumber yard with a tape measure you will notice this is not the exact size of a 2x4 or 2x6 but instead the 2x4 will measure approximately 1-1/2 x 3-1/2 and the 2x6 will measure 1-1/2 x 5-1/2. This is the difference between nominal sizing vs actual or rough sawn cut sizing. When the piece of lumber is cut into an actual 2x4 at the mill, it must then go through an additional milling process that involves a planer in order to smooth out or "dress" the surfaces of the lumber. This smoothing process reduces the actual size of the 2x4 down to a 1-1/2 x 3-1/2 putting that piece of lumber in the nominal sizing category. A 2x4 piece of lumber that has not been through the surface smoothing process and has only been cut once will measure out to a true 2x4 putting it into actual size or rough sawn cut size. These rough sawn cut pieces of lumber will have a very rustic look to them which is desirable for some projects. However, when building structures that require post bases, column caps, and beam support brackets it is very important you know which type of hardware to use based on the type of lumber that is supplied for the project. Simpson Strong Tie ABU66Z is a ZMAX coated post base designed to fit the nominal size 6x6 piece of lumber; this means that the ABU66Z post base is actually designed to fit a 5-1/2 x 5-1/2 piece of lumber. However, if you have a 6x6 piece of lumber that is rough sawn cut; you would use a ABU66RZ post base because it's designed to fit a true rough cut 6x6 piece of lumber. Also, if you needed a face mount hanger for a 2x6 that is nominal size--you could use Simpson LU26. If that 2x6 is rough cut sizing, you would have use Simpson LU26R. 

Below is a table for common nominal sizes and their actual size.

Nominal        Actual

1x2                3/4 x 1-1/2

1x3                3/4 x 2-1/2

1x4                3/4 x 3-1/2

1x6                3/4 x 5-1/2

1x8                3/4 x 7-1/4

1x10              3/4 x 9-1/4

1x12              3/4 x 11-1/4

2x2                1-1/2 x 1-1/2

2x4                1-1/2 x 3-1/2

2x6                1-1/2 x 5-1/2

2x8                1-1/2 x 7-1/4

2x10              1-1/2 x 9-1/4

2x12              1-1/2 x 11-1/4

3x6                2-1/2 x 5-1/2

4x4                3-1/2 x 3-1/2

4x6                3-1/2 x 5-1/2

6x6                5-1/2 x 5-1/2