The sizes of mattresses use non-numeric labels such as a "king" or "full", but are defined in inches. Historically most beds were "twins" or "doubles" but in the mid–1940s larger mattresses were introduced by manufacturers. These were later standardized as "queen" and "king", and first made a significant impact on the market in the 1950s and 60s. Standard mattress depth ranges from the "standard" size of to "high contour" of up to . Below are the standard ISPA widths and heights in the United States and Canada.