These sheets are definitely well made and will definitely stay in place on our split king adjustable bed. Why didnt I give it 5 stars? The cotton feels rather coarse. Perhaps I am unfairly comparing it to Lyocell sheets? Please comment and correct me if I am wrong and I will revise review.
All in all these are excellent quality, US made sheets. They fit our Sleep Number M12 with Prodigy adjustable base bed just fine.
UPDATE 2/23/16: Was contacted by Beth and they are resolving this issue about the softness of the cotton that went above and beyond my expectations. A+++ and earned a 5 star rating!