Driftmaster rod holders are designed to hold you rod securely, yet release the rod easily when you get a strike.  Just grab the rod and pull up to release the rod and set the hook.  
Pro series rod holders have a 1/2" threaded stem and require pro series bases.  They are 1 3/4" wide to handle large diameter rod handles.  Pro series rod holders are suggested for bigger fish and heavy rods.
Right thread rod holders handle most fishing methods.  We use serrated flange nuts to secure the rod holder to the base.  These nuts help to prevent the rod holder from turning under pressure. 
Left thread rod holders prevent the a rod aimed out of the left (port) side from turning left during a hard pull.  Fast trolling, pulling umbrella rigs or planer boards can cause this.  Left thread rod holders require left thead bases and come with hex nuts.
All right thread rod holders come with a serrated flange nut and left thread rod holders come with a standard hex nut.