Shitajiki, Writing board, Pencil board. These go by a number of different names, and come in a large number of variations. I’ve been disappointed by many in my search for some to make available here. Many are quite hard and really only provide protection for the page underneath.
These A4 Shitajiki are just under 1mm thick, but fairly soft, to provide a cushion while writing and protecting the sheet underneath at the same time. The grid pattern on these is an 8mm spacing and the reverse side is blank.