What does the 'interfacing' mentioned in the shirt description refer to?

Interfacing is an 'inner fabric that holds the framework of clothing.' It cannot be seen from the outside, and by adding a layer inside the outer fabric, it keeps that area from losing its shape. In shirts, it is mainly used in the collar and cuffs, which need to maintain their shape, as well as the front placket where the buttons are attached.

This shirt is reinforced with soft silk interfacing so that its shape is stably maintained^[1]. Various materials are used for interfacing, and the stiffer types that create sharp angles make the shape clearer, but feel harder against the neck or wrists. Silk fibers themselves are fine and flexible, making it closer to retaining a soft touch against the skin while still maintaining the shape.

Since interfacing is an area that cannot be touched from the outside once applied, the difference shows the longer you wear it. In particular, with fusible interfacing, repeated washing and ironing can cause the bonded surface to lift and create fine wrinkles on the collar surface, so it is generally safer to avoid high-temperature ironing and intense drying.