The feasibility is examined of devising a scheme for operating a joint keyword system in a given subject area using several independently compiled thesauri. It is suggested that if the individual keywords of each participating thesaurus represent identifiable concepts, and these concepts are given unique code numbers, then the code numbers enable the keywords of any participating thesaurus to be converted into the appropriate keywords of any other participant. Incompatibilities between keyword systems arise from differences in the selection and form of keywords. Eleven types of incompatibility are identified and a method of reconciliation is proposed for each. The types of incompatibility that can arise in a single language include those that arise between one language and another, so that a multilingual joint system presents no additional problems.