This is a fun little "single" feather fly shown to me by Ilene Hirsch of "Flys by Ilene" here in Oregon.  They are simple, quick and very effective when light colored Mayfly spinners are present.

Start thread at mid-shank and lay down a thread base to hook point then back to mid-shank again.  Take a single mallard flank feather (small) and notch the tip.  Fold back all of the barbs towards the base of the stem, except one barb on each side.  Holding the barbs folded to the stem, tie the feather in at mid-shank. Clip the stem and pull the remaining barb tips up to create a wing post (clip excess if you used too big a feather, like I did on this one).  Wrap appropriate colored hackle making two or three wraps behind and 5 or 6 wraps in front of the post.  Whip finish and cement head.

In the fly shown I used tan thread and grizzley hackles to match the drab colored mayflies in my area, but I'm sure you can come up with thread and hackle colors to more closely match the mayflies from differnt parts of the country to make this fly more effective for you.

This fly will ride with the hook point extending at an angle below the surface which greatly increases the chances of hookups.  They practically hook themselves on this fly.