Date-eri (neckband for show)
The date-eri or kasane-eri is the decoration neckband to put between the collar for under kimono and that of kimono. It is mostly used in formal kimono dress, but also worn as a simple decoration in casual dress recently.
Originally high-class kimonos were worn in layers, but it went simplified and then we began to use the Date-eri to show as if we wear layers of them. There is no particular rule for the color or the material, so you can choose one according to your taste.
No particular rule how much to show either, often 1-2 millimeters or so in the width. Note that it is not always necessary to wear it even in formal dress. Many don’t wear Date-eri but rather make gorgeous Han-eri (quality collar for an under kimono) or just prefer simpler way of dressing. It is just cloth, so you may be troubled how to fasten it when you wear it for the first time; it can be fixed generally with the clips or the likes, or sewn on the Han-eri, too. In doing this you have to fix or sew it only by the part behind the neck and arount the shoulders so that you can adjust how much to show from the front neckline in dressing.