Each main UDC class may also contain tables called special auxiliaries (or special auxiliary numbers), which express aspects that are recurrent, but in a limited subject range. These are usually facets of concepts related to operations, techniques, processes, materials, agents etc. They are listed only in particular sections of the main tables. Special auxiliary numbers can be recognized as they all begin with one of these three specific symbols/indicators: .0 (point nought), - (hyphen) or ' (apostrophe). Any UDC number beginning with any of these symbols can be combined with any other UDC number in its designated area of application.