Keeping sheet corners on the mattress

I recently found a way to keep my sheets on my bed.  Because I have acid reflux and the bed frame is tilted 5" (with blocks under the head of the bed), the older, slightly smaller sheets tend to come untucked at the top.  It's a pain to pull the sheet corners back under the mattress every night!

I got to thinking about it, and did some measuring on the fitted sheet.  I decided that if I made a triangle with sides of 25", hemmed the long side, and sewed it into the corner of the fitted sheet that it would hold the sheet onto the bed.  So I cut up part of an old sheet into two 25 inch squares, cut them diagonally, and hemmed the long side of each one.

Then came the "difficult" part, sewing the triangle into the corner of the elasticized bottom sheet.  Sewing this is as if you took two pieces of fabric, overlapped them at one edge, and put them under the needle and sewed them together.  Except that the bottom fabric has a finished edge (the elastic casing).

Step One:
I settled the fitted corner right side down under the needle of my machine, with the rest of the sheet flowing down to my left side.  I want to sew on the "inside" of the fitted sheet corner.

Step Two:
I placed the triangle, right side down, center corner pointing to the left on top of the edge of the fitted sheet corner along the elastic casing seam line.  I diagonally sewed down an inch of the triangle corner to the left of the elastic casing line across the seam at the sheet corner, with the rest of the triangle hanging off to the right.  I left the needle down, in the fabric, to help hold it in place for the next step.

Step Three:
Then I laid one side of the triangle pointing from the needle to me, on top of the fitted bottom sheet - then pulled the bottom sheet elastic out tight towards me with one hand, held onto the fabric at the back of the needle with the other hand, and began sewing.  This way, the triangle could lie flat on the stretched-out edge of the bottom sheet.  I was careful to sew a bit to the sheet side of the elastic casing so I wouldn't catch the elastic in the stitching line.  I backstitched at the far tip of the triangle.  I removed it from the machine and cut threads.

Sept Four:
I then slid the sheet and triangle around behind the needle so now I had the sheet corner on the right side of the needle (and hanging off the back of the machine) and the triangle on the left.  I set the needle into the same starting point area at the corner of the triangle and stitched a few stitches to hold it all together so I could once again pull the triangle (point towards me) over the top of the stretched out (pull gently!) bottom sheet and sew just to the sheet side of the elastic casing.  I backstitched at the tip once again, and cut threads.

I then repeated this three more times for the other three corners.
I found that once the triangle corner is sewn down, it would wrinkle as I maneuvered the long point towards me on top of the fitted sheet.  I sewed it down over the wrinkles, and it didn't affect the finished piece.  The goal is to hold the corner on the bed, not to worry about the wrinkles sewn into an area no one can see.

Now when you hold the sheet up and let the triangle fold under, you can see that you have added the triangle to the inside of the sheet corner and the elastic casing can still do its job normally.  All the seams will be hidden inside the bottom sheet next to the elastic casing.

These extra triangles really keep the sheet in place!!  I think they would be especially great if the elastic is all stretched out, but the sheet itself still fits the bed. No need to put in a new elastic band.