a1 Department of Pure Mathematics, University of Liverpool L69 3BX
Alexander  showed that an oriented link K in S3 can always be represented as a closed braid. Later Markov  described (without full details) how any two such representations of K are related. In her book , Birman gives an extensive description, with a detailed combinatorial proof of both these results.
(Received February 15 1985)
(Revised July 11 1985)