In Chapter 21, Poirot makes a connection between the victims? What is this connection? The A.B.C. Murders

In Chapter 21, Poirot confronts Thora about what Lady Clarke had said, and eventually she remembers that there was someone trying to sell her pantyhose on the day of Carmichael’s murder. She says that he was an unforgettable type of person, though. This stirs something in Poirot who remembers now that in all the cases there was something about pantyhose. In Andover, Mrs. Ascher had a new pair in her apartment. In Bexhill, Betty’s mom had bought her a pair, and in Churston, Thora had been asked if she wanted a pair, as well.