For a proper understanding of the type concept it is recommended to read the general introduction. A holotype is a single physical example (or illustration) of an organism, known to have been used when the species was formally described. It is either the single such physical example (or illustration) or one of several such, but explicitly designated as the holotype. Under the ICZN, a holotype is one of several kinds of name-bearing types. An isotype is a duplicate of the holotype.