TCL’s three-fold phone: Bend and bend again

I was a child when I first heard the word concertina. I didn’t know what it was and barely know now, but in our family, it meant that my parents both stood on either side of the bed and took a good 15 minutes to fold up their sheets and blankets as part of their make-the-bed routine. Carefully aligning a sheet underneath and a blanket on top, they would pull carefully upwards, downwards and back upwards again, forming a layered fold that looked like part of it might have been borrowed from the accordion-like musical instrument after which the fold is named.