Can you see how I'm illustrating it? I measured 5" down either side of each corner seam. Then I connected those two spots with a 2.5" piece of elastic. Now there's a kind of "hood" that slips over the corner of the mattress. It took me about 10 minutes to fix that sheet.
And when I looked up from the clothesline, I saw the scarlet bush. Flaming beautiful. I've lived in this house eight years and I don't remember noticing this bush before. I want to look around more carefully, with more appreciation.