In the United Kingdom and Ireland, beds are measured according to the size of mattress they hold, not the dimensions of the bed frame itself; bed frame sizes are not standardized and may differ between manufacturers. Listed below are the typical bed sizes from the National Bed Federation, which is the trade association for the majority of British and Irish bed manufacturers and their suppliers. Most NBF manufacturers use designs dimensioned in feet and inches with the metric indicators not being exact equivalents. There can legally be a tolerance of up to ± between the quoted measurements and the size of the mattress itself.