Roll, Pitch, Yaw are terms to describe the rotation of a vessel about its three axes due to wave action upon the vessel. Heave, Sway, Surge are terms to describe linear motions of a ship, also due to wave action.
- As the vessel rotates about its long axis, so that it 'leans' to the left or right, that motion is referred to as roll.
- As the vessel rotates about the axis through the beam so that the bow moves up and down, that motion is referred to pitch.
- As the vessel rotates through its vertical axis so that the bow moves back and forth between port and starboard, that motion is referred to as yaw.
The sailor must learn to stand and walk on a moving deck due to roll and pitch, but, the quartermaster must continuously correct for yaw with the rudder.
Heave, surge, sway are terms to describe the linear motion of a vessel on its three axes due to wave action or other forces not including the motion due to sailing and leeway.
- As the vessel experiences linear motion to port (larboard) or starboard, the motion is referred to as sway.
- As the vessel experiences linear motion fore or aft, the motion is referred to as surge.
- As the vessel experiences linear motion up or down, the motion is referred to as heave.
A ship in waters with wave action experiences motions of all six degrees of freedom almost constantly. As a result, it becomes difficult to walk on a deck until one develops his or her ' sea legs', or the ability to deal with it. This can also result in 'seasickness', many times more likely if one is below decks such that one senses the motions without the visual cues which match the sensory cues.