The nagajuban is an underkimono that is similar to the kimono in the shape, except that it is shorter. It has a collar sewed on, that is supposed to show under the kimono. This is often white, but can be very colorful. The nagajuban prevents the kimono from getting dirty or smelling and is in that sense just a kind of underwear. But nagajuban can be as elaborately decorated as the outer kimono, although it is often red, white or pink.
Of course there are several kinds of Naga-Juban. Some are for the use on the ceremonial occasions with mourning kimono or bridal Furisode, Others are for rather casual occasions with Tsukesage etc. Rinzu, Chirimen, Muslin are usually used as the material of Naga-Juban. And with summer kimonos, Sha or hemp are mainly used. Each material has each own characteristic. It can be said that Naga-Juban is a hidden smartness.