Common sealing orientation and directions for hydraulic seals and pneumatic seals include internal or rod seal, external or piston seal, symmetric seal, and axial seal.  Rod seals are radial seals.  The seal is press-fit into a housing bore with the sealing lip contacting the shaft. Also referred to as a shaft seal.  Piston seals are radial seals.  The seal is fit onto a shaft with the sealing lip contacting the housing bore. V-rings are considered external lip seals.  Symmetric seals are symmetrical and works equally well as a rod or a piston seal.  An axial seal seals axially against a housing or machine component.  Sealing direction pertains primarily to hydraulic and pneumatic seals that are used in applications with axial motion, such as cylinders and pistons.  The action can be single or double.  Single acting, or unidirectional seals, offer an effective seal in one axial direction only.  In order to seal in both directions for a reciprocating motion, more than one seal must be used.  Double acting, or bi-directional seals, are effective when sealing in both directions.  FEROS