Compression springs are wound or constructed to oppose compression along the axis of wind. They can be wound in constant or variable pitch along their length, be constructed of stacked spring elements, and can be straight, conical, or have a variable diameter.  The most important parameters to consider when searching for compression springs are the dimensions.  These include outer diameter, inner diameter, wire diameter, free length, and solid height.  The free length is the overall length of a spring in the unloaded position.  The solid height is the length of a compression spring when under sufficient load to bring all coils into contact with adjacent coils.  Other important considerations include spring rate and design units.  The spring rate is the change in load per unit deflection, generally given in pounds per inch or Newtons per millimeter (N/mm).  Design units can be specified in English or metric units.  Some manufacturers may specify both.