Parallels are used in machining operations, be
it milling, drilling, turning or sometimes grinding. The most common
use is to support work when it is in a vise or clamped to the
machine bed. If a workpiece is too small to be machined in a vise
without it being in contact with all three faces of the vice -
parallels can be used either side to give clearance from the vise,
and to give support from underneath to eliminate the workpiece being
pushed down by the force of the cutting tool.