A thrackle is a graph drawn in the plane so that its edges are represe
nted by Jordan arcs and any two distinct arcs either meet at exactly o
ne common vertex or cross at exactly one point interior to both arcs.
About 40 years ago, J. H. Conway conjectured that the number of edges
of a thrackle cannot exceed the number of its vertices. We show that a
thrackle has at most twice as many edges as vertices. Some related pr
oblems and generalizations are also considered.