The rind is affixed to the top of the main shaft or spindle and supports the entire weight of the runner stone, which can be as much as several tons. The rind is necessary because the grain is fed through the runner stone's central hole, so the spindle cannot be inserted through it like a cartwheel on an axle.
The face of a runner stone usually has a carved depression, called the "Spanish cross", to accommodate the millrind.
The millrind occasionally appears as a charge in heraldry, in which it is often known by the French name fer-de-moline ("iron of a mill"). Like real millrinds, the fer-de-moline is highly variable in form. The 16th century writer Bossewell characterized it as a symbol fit for judges and magistrates, who keep men on a straight course just as a millrind does with a runner stone. However it is more often found as a rebus in the arms of families with names like Miller or Milne.
Another charge based on the millrind is the cross moline, which takes the form of a cross with bifurcated ends (sometimes with a pierced centre and sometimes without). In early blazons the term fer-de-moline often refers to the cross moline.