Two ideas from classical knot theory are (i) the Tait graph (1877) which defines a winding skeleton for double-helices forming knots and (ii) the Seifert genus of a knot (1934), which is the (minimum) genus of an orientable surface whose boundary is the knot. Both concepts meet when thinking about the most stable "folds" of single-stranded RNA: branched or unbranched right-handed antiparallel double helices. To our surprise (ignorance?), it turns out that Seifert's construction gives exactly what we need: a homochiral all-antiparallel double-helical fold. Further, only 32 of the simpler 200-odd knots are realisable as stable folds. (And explicit recipes for cooking up ssRNA which is expected to fold into those knots are now available. Designer knots  - at the nm scale - are here!)