～畳
～じょう · ～jou · JLPT N2 · noun
mat
畳 also spells tatami, from 畳む tatamu, to fold or pile up
Rooms in Japan are measured in mats: 六畳 rokujō is a six-tatami room, about 10 m². The kanji 畳 is the same one in 畳む tatamu, to fold, because tatami were once mats folded and stacked when not in use.