A porcelain onlay is a much like a smaller or partial version of a crown. Porcelain margins can be hidden in nearly any surface of a tooth to blend in perfectly. The preparation of the teeth for these restorations can vary greatly, often allowing for the decay or anatomy of the tooth to dictate the size and shape. Because of this, healthy tooth can be preserved that would otherwise be prepared away for other types of restorations. Onlays come in many different shapes and sizes, and are an alternative to many different treatments, most commonly large fillings or crowns.