This is employment in which a free worker sells his or her labour for an indeterminate time (from a few years to the entire career of the worker), in return for a money-wage or salary and a continuing relationship with the employer which it does not in general offer contractors or other irregular staff. However, wage labour takes many other forms, and explicit as opposed to implicit (i.e. conditioned by local labour and tax law) contracts are not uncommon. Economic history shows a great variety of ways, in which labour is traded and exchanged.