Parallels are steel bar stock accurately machined so the opposing edges are parallel. Sold in pairs with identical dimensions, they’re useful for many machining operations.
Parallels can support workpieces exactly parallel to the table but raised to provide clearance if a cutting tool extends through the bottom of the material. Parallels can also support pieces parallel to the base of a vise, raising the piece high enough in the jaws so the cutting operation doesn’t damage them. There are many more uses too.
This relatively inexpensive set from Chicago includes ten matched pairs of parallels: 1/2″, 5/8″, 3/4″, 7/8″, 1″, 1-1/8″, 1-1/4″, 1-3/8″, 1-1/2″, and 1-5/8″. Chicago machines the 1/8″ thick by 6″ long parallels to within 0.0002″. Each parallel is hardened to HRC 53-57 and has two holes drilled into it. The set comes in an indexed case.
For just being a few scraps of machined metal, parallels aren’t cheap — the set by Chicago brand runs about $55, and that’s on the low end of the scale. You can pay upwards of $1,000 for a set.