マルアイ チェック柄封筒 青 125x240mm

The product you are referring to is a Maruman (マルアイ) check-pattern envelope (チェック柄封筒) in blue, with dimensions of 125x240mm. Maruman is a brand known for its stationery products, including notebooks, paper, and various types of envelopes.
If you have specific questions about this envelope or need more information about its usage, purpose, or features, feel free to ask!