Timbers are a classification where the material is sized to a nominal dimension of 5 inches or greater (5x5 and larger). Timber grades are classified into two basic groups according to the intended use: Beams & Stringers, and Posts & Timbers.

Beams & Stringers - nominal 5" & thicker with the width more than 2" greater than thickness. Grades are: 

SELECT STRUCTURAL
No. 1
No. 2

No. 3 (no design values assigned) 

DENSE SELECT STRUCTURAL (in DF-L only)
DENSE No. 1 (in DF-L only)
DENSE No. 2 (in DF-L only) 
(Note: DENSE grades are not as widely available.) 

Posts & Timbers - nominal 5" x 5" with the width not more than 2" greater than thickness. Grades are: 

SELECT STRUCTURAL
No. 1
No. 2 

DENSE SELECT STRUCTURAL (in DF-L only)
DENSE No. 1 (in DF-L only)
DENSE No. 2 (in DF-L only) 
(Note: DENSE grades are not as widely available.)