Susan, SF means single fan and DF means double fan. Double fan implies that the two fans will be connected but that is not always the case. Some sellers will list as 2F or two fans implying that the two fans may not be connected and in essence be 2 single fans. You often do not know how the daylilies will come apart once dug and divided. The safe listing is 2F, I think, though I often say "double fan" for two fans.
Doris
