noun
medium-sized paper; notebook paper size
Refers to a specific paper size approximately 15×21 cm, commonly used for notebooks and writing pads. Not a standard ISO size.
このノートは合い判サイズで、持ち運びに便利だ。
This notebook is in the medium-sized format, convenient for carrying around.
noun
medium-sized photo print; photo print size
Refers to a specific photo print size approximately 10×13 cm, a common format for snapshots and family photos.
昔のアルバムには合い判の写真がたくさん貼ってある。
In old albums, many medium-sized photos are pasted.
合い (ai, 'together' or 'mutual') + 判 (ban, 'size' or 'format'). The exact origin is uncertain, but it likely refers to a 'medium' or 'in-between' size compared to 大判 and 小判.