I was reading someone's comment on another thread and thought it somewhat short-sighted. Or, maybe it's me and I'm being too far-sighted. Anyway, he was talking about the wings on traditional wet flies and said the following:

"Flank feathers is a term that has been used for some reason for wings on a Traditional winged wet. When you cut a segment out of the flank feather it's called a slip. You need a flank feathers that are paired left and rights.
To me when you use the term traditional winged wet, it means duck or goose quills, Turkey, Guinea Hen, barred Wood Duck, Bronze wood duck or Teal and lastly plain old paired left and right mallard flank."

Now, the term 'traditional' may start in one era for some people, other eras for others and certain places for yet others. The tradition of wet fly usage and patterns here in the US, had its beginnings well before the turn of the 20th century. Regardless, sources I've read included flank feathered wings in the term traditional and that the way they are tied in did not have to be 'slips'. They could be rolled as well.