Sokutai is a formal costume for those from the Emperor to the court nobles in and after Heian period (Heian costume). Ikan is called 'tonoi (nighttime) costume', whereas sokutai is called 'hino (daytime) costume'.
How sokutai is worn
A sokutai is put on in the following order: first a hitoe (a single layer of a kimono that serves as an undergarment) is put on; next, an akome (a layer of clothing worn by nobles beneath their robe but over their undergarments) is put on; next, a shitagasane (long inner robe) is put on; next, a hanpi (short-sleeved or sleeveless undergarment) is put on; next, a ho (outer robe) is put on; and finally a leather belt is put on around the waist on top of the ho. There are two types of hakama, oguchi-bakama and omote-bakama: first oguchi-bakama is put on, then omote-bakama is worn on top of the oguchi. A crown is put on the head and shitouzu is put on the feet. Tatou and hiougi are put in a space on the chest and a shaku is held in a hand. Kugyo (the top court officials) and tenjobito (a high-ranking courtier allowed into the Imperial Palace) had a decoration item called gyotai hung at the waist.
Although there were some differences between the sokutai worn by civil officers and that worn by military officers, military officers with a rank of Sanmi (Third Rank) or higher and civil officers both wore a hoeki no ho (robe with a round collar, stitched sides, and a ran that was worn by the emperor and high-ranking officials) and a suiei (a kind of "tail" that was attached to the back of formal headwear) attached to their kan (formal headwear). Military officers at the fourth court rank or lower used ketteki no ho (open sleeve steams outer robe) and kenei for a cap (an item which is a string of a crown rolling up inwardly and hold with black stained hasamigi). In addition, military officers, officers in Ministry of Central Affairs and civil officers at the rank of Sangi (councillor) or higher who were permitted by the Emperor had a sword at their waist. In this case, the sword was fastened with hirao (cord of sword) and was tied up to the waist.
The ushiro-migoro (back main panel) of shitagasane (long inner robe) was made long and was trailed like a long tail when it was put on. This portion was called 'kyo (hem)' and was a remarkable feature of the figure of a person in sokutai. When the length of kyo started to indicate the rank of persons, shitagasane and kyo were separated from each other because shitagasane itself became so large (bekkyo, separated kyo). In this case, after putting on shitagasane, the ends of the belt extended from kyo were brought in front and were tied up. However, as to sokutai worn by the Emperor and the princes, shitagasane and kyo were connected with each other. There was also short kyo called saijaku, and this was used for sokutai for jigenin (a lower rank of ancient Japanese nobility).
In winter civil officers wore sokutai without hanbi. This was because the both sides of the chest of hoeki no ho were closed, therefore, it was not seen whether hanbi was worn or not. Hanbi was worn in summer because hanbi could be seen through (because the cloth of ho was thin), however, later during the early-modern times, hanbi was abolished even in summer. According to an episode described in "Imakagami" (The mirror of the Present), in a scene in a casual atmosphere, when some people took off ho from their shoulders, the people exposed their shitagasane and only Norimichi FUJIWARA orderly wore hanbi and the people around were deeply ashamed of themselves. This indicates that hanbi should be worn even in winter in principle. Emperors are not supposed to omit their hanbi.
Ikan (traditional formal court dress) was originally a costume for night duty (called tonoiginu) in the Imperial Court, however, it started to take root as a working costume in the Imperial Court, then sokutai became to be worn as a formal costume in ceremonies.