British Standard definitions for spacers
BS 7973 defines the various types of spacers available as follows.
spacer - component which is placed to maintain cover between the reinforcement and formwork or blinding and which provides cover or support for reinforcement at a single point on one or more faces of the formwork or blinding.
end spacer - spacer designed to fit on the ends of reinforcement to provide end cover
line spacer - spacer normally made of either plastics or cementitious bars, normally of constant cross section, used to support horizontal layers of reinforcement or horizontal continuous steel chairs.
rib spacer - spacer used where there are no links within the ribs and which are designed to support one or more main bars in hollow pot, waffle, trough and ribbed slabs and to provide spacing betwen the sides of the hollow pot, waffle, trough and rib forms.
wheel spacer - spacer having circular section, perpendicular to the bar to which it is attached, used for maintaining cover in vertical members to the reinforcement nearest the surface of the concrete.
wire-on spacer - spacer that is attached to the bar by the use of tying wire. Note: wire-on spacers are usually cementitious.
clip-on spacer - spacer in which the clip-on action is an internal part of the design of the space. Note: clip-on spacers are usually plastics