They take more of a beating than any other part. Several designs are shown.
Door hinges are also critical hardware elements.
Choose these unless you want the period look of surface hardware. Solid lumber is sometimes used, but is usually reserved for doors and drawer faces. Hardwood plywood is surfaced with attractive wood veneers on face and back. The higher the face grade, the more you'll pay. Particleboard costs less, weighs more, and is both weaker and more prone to warping and moisture damage than plywood.