• 畳たたみ — tatami mat
• 一畳いちじょう — one tatami (mat)
• 半畳はんじょう — half-tatami; small mat
• 四畳半よじょうはん — four-and-a-half tatami (room size)
• 折り畳おりたたみ — folding; foldable
• 畳み掛たたみかける — to press; to pile on (questions, etc.)

The traditional form of 畳 shows layers stacked up — straw piled and bound — which is exactly how a tatami mat is made. From “layers” came the senses “to fold” and “to pile up,” and by extension the counter reading ジョウ for measuring room size.