This was the first in a series of four articles by E. Buony. It described two de Vivie 2 speed systems. Each used two chainwheels and two rear sprockets. By using chainwheel and sprocket pairs that have the same total number of teeth (one set of suggested pairings was 23T driving 10T, and 20T driving 13T) the same length of chain could be used for each pairing. You changed gear by stopping and shifting the chain from one chainwheel/sprocket pair to the other.
The two de Vivie systems differed from each other in that one had a chainwheel and sprocket on each side of the frame (this was Paul de Vivie’s preferred layout). The other had a double chainwheel and a double sprocket all on one side of the frame.
(Source gallica.bnf.fr/ Bibliothèque nationale de France)