Tatami mats were originally a bed and in the aristocratic society during the Heian period strict usage showing the status of the person who was seated there was established as a person with higher status used larger, thicker and more tatami mats.
畳は起源的には寝具であったが、平安時代の貴族社会では、そこに座る人の身分地位を表す厳密な用い方が定められ、身分の高い人ほど座る畳も広く厚さも厚く、畳を重ねて使用した。