Adjustable razors are those safety razors that have built-in mechanisms, frequently a twistable knob at the base of the handle, to adjust the razor’s head and expose more blade. The more blade that’s exposed, the more aggressive the shave and, in theory, the better the shave overall for the experienced shaver. The numbers indicate how aggressive the shave will be. Typically, one is the least aggressive shave, with the least amount of blade exposed, and six is the most aggressive, with the most amount of blade exposed.