To prepare a rest seat, 1.5 mm of the marginal ridge is reduced to provide adequate thickness for support and strength without compromising tooth structure. This reduction ensures proper seating of the rest and distributes occlusal forces effectively. Over-reduction weakens the tooth, while under-reduction fails to provide enough space for the rest, leading to instability. Maintaining this precise measurement preserves tooth integrity and optimizes prosthesis function.

Reference: Prosthodontics, Boucher’s, 5th Edition.