Oil seals and grease seals have a flexible lip that rubs against a shaft or housing to prevent the leakage or ingress of fluids and dirt. For proper installation, the seal lip should point towards the medium being contained. Some oil seals and grease seals have a spring to help keep the lip in contact with the shaft.  The sealing orientation and direction is important to consider for oil seals and grease seals.  The orientation and direction can be internal or rod seal, external or piston seal, symmetric seal, or 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.