The movement of the ground during an earthquake can be separated into its three space components, that is to say, the waves are the sum of a vertical component, a nord-south one and an east-west one. To this measurements three seismometers are requiered, as shown in the pictures.
Vertical motion can be recorded by attaching the mass to a spring hanging from the frame; the bobbing mass will inscribe a record. For measurements of the sideways motion of the ground, the mass is usually attached to a horitzontal pendulum, which swings like a door on its hinges.