Partial dentures replace only some teeth on either the upper and/or the lower jaw and are held in place by retainers or clasps. There are two types of partial dentures: An acrylic denture, utilises spring wire clasps for retention, and are easily and quickly constructed. They are generally thicker and occupy a greater area in the mouth. They are usually seen as a short term, economic or emergency solution. The second is a precision cast metal framework, cast in either cobalt-chromium or titanium. They have a superior fit, are comfortable, and are thinner and stronger. Teeth can be added to them and their life is measured in decades. A partial denture may be used to balance the opposing arch teeth to prevent over-eruption and also prevent the remaining teeth drifting out of position making future tooth replacement considerably more difficult. By wearing a partial denture preservation of the remaining natural teeth is enhanced and wear and tear of remaining natural teeth is reduced because chewing and grinding forces are more evenly balanced.