All casters are mounted to cases with an additional 1/2″ or 3/4″ caster board. Note that in the picture descriptions below, we specify “Shore” followed by “A” or “D” and a number. These values represent a “Shore Durometer” rating. There are two common rating scales, using slightly different measurement systems, which are represented by “A” or “D” where the A scale is for softer materials and the D scale is for harder ones.Higher numbers in the scale represent a greater resistance to indentation, and thus harder materials; lower numbers represent less resistance to indentation and softer materials.