I use 3 . 6/8 for the 1st pass , 5/8 for the 2nd , and a 4/8 or 9/16 for the upper and lower lip area . Why ? Because I shave about every 3 days , and IMHO a 6/8 does a better job on the heavy stubble than a 5/8 . I use a 5/8 for the second pass because I need to do something with my 5/8 razors , and having a freshly stropped razor for the 2nd pass is nice . Not absolutely necessary , but nice .