Clients are always saying that they want their dog’s hair to be x length.  On pups like poodles and Bichon’s where the hair stands up, length and depth of the hair are basically the same.
However, if you have a flat coated dog like a Chihuahua, these terms mean two very different things. Accomplishing what the customer wants is done in very different ways.  Customers always say “length.”  A Chihuahua might have hair that is 2 inches long, but cutting using a #2 comb with the lay of the hair will take almost nothing off the length.  Cutting with the lay of the hair on a flat coated dog is really trimming the depth of the hair.  To accurately cut the “length” of the hair one must use the comb and cut in reverse, or against the lay of the hair.  https://www.dogforums.com/threads/with-or-against-the-grain.49538/