1. Three-component recording involves the use of three orthogonal geophones, usually vertical and two horizontal. Four-component recording adds a hydrophone. Nine-component recording involves the use of three orthogonal sources into three orthogonal spreads of geophones. Occasionally, only two components are recorded, sometimes in the two horizontal directions, sometimes vertical and inline. See Tatham and McCormack (1991). 2. Recording multiple modes of particle motion such as pressure, velocity, acceleration.