In addition to these barriers many occupations have strong professional bodies (e.g. accountancy) or strong trade unions (e.g. the closed shop) which regulate entry to an occupation. The higher these barriers are to mobility in an occupation, the more inelastic will be the supply of labour to that occupation. The supply of brain surgeons is likely to be highly inelastic due to the qualifications and skill needed and the length of training. If there is a shortage of brain surgeons, it would be very difficult to increase supply quickly because of these factors. Unskilled labour is likely to have a much more elastic supply.