Ikan (traditional formal court dress) (衣冠)
Ikan is the work clothes of nobles and government officials in the Imperial Court after the Heian period.
Originally, work clothes at the Imperial Court were chofuku (clothes for the people who come to work at the court on a regular basis wear) and sokutai (traditional ceremonial court dress), which was converted from chofuku. However, sokutai was not suitable for Tonoi (night guard) since it was sekitai (leather belt) and hence constrictive. Therefore, ikan was created as 'Tonoi sozoku' (a costume of Tonoi, the night guards wear).
The basic structure of ikan is the same as sokutai, but charactalistically, its underclothing is drastically simplified from sokutai. While sekitai is worn over ho (round-necked robe worn by members of nobility and the Imperial Court) in sokutai, in ikan, kukehimo (narrow sausages of silk cloth) of common cloth is worn over ho, and hakama was loose sashinuki (pleated trousers for formal wear).
To wear ikan, you first wear underwear, then sashinuki, hitoe (a single layer of kimono), and ho. Then, you wear suiei no kanmuri (a kind of cap included into official uniform which is a type of having Ei, an attachment of cap hanging back of head) and hold a folding fan. You hold shaku (mace) only when you wear ikan to visit a shrine. In addition, when you carry a sword, hirao (cord of sword) is not used.
Unlike sokutai, there is no difference between ikan for civil officers and military officers, and military officers also wear hoeki no ho (robe with a round collar, stitched sides and a ran, worn by the emperor and high-ranking officials).
Rules about color combinations and patterns indicated ranks.
Changes over time
Over time, sokutai became the ritual accouterments and ikan became established as the work clothes in the Imperial Court. In addition, you didn't have to obtain imperial sanction for sandai (a visit to the Imperial Palace) if you were wearing ikan. As the 'Dairi koban' (night guard in the Imperial Court) system was established in the early-modern times, noshi (everyday clothes for nobles) and ikan became common as daily work clothes other than ceremonies. In the Edo period, they sometimes wore ho directly over shiro kosode (a kind of white underwear) without wearing hitoe under ikan or noshi, or 'sashiko' (a kind of quilted trousers) or kiri-bakama (short fringe hakama) was worn instead of sashinuki for ordinary serving. Therefore, ikan clothing wearing hitoe is also called 'ikantan' or 'ikanhitoe' (kimono and ancient head-dress with a layer). In kuge-nikki (noblemen's diaries) in the early-modern times, there are articles that describe someone wearing sashiko while serving who suddenly needed to change his clothes to sashinuki to say thank the emperor. Even in this period, in ceremonies that were not government affairs such as New Year Imperial Poetry Reading Parties or New Year Gagaku (ancient Japanese court dance and music) Recitals or private ceremonies such as genpuku (celebrate one's coming of age), they wore official ikan or noshi layering hitoe or kinu and sashinuki.
Since 1872, European clothes have become the formal attire and ceremonial dresses in the Imperial Court and the government, so now ikan is used by Shinto priests in shrines as formal attire, besides the imperial families and ceremonial staffs wear ikan as ceremonial attire in court rituals and ceremonies.
In literature in the medieval period, sokutai and ikan were not precisely distinguished, but the word 'ikan' may be used to simply mean formal attire of nobles as in the 'ikan tadashiki' (properly wearing ikan). In diaries and writings on records of the past, the word is usually used strictly, and in documents of theory about classic Japanese poem and legends of temples and shrines, the word is often used in the vague way. Also in common parlance, general term of the figures wearing ho is sometimes called 'ikan-sokutai,' but fundamentally, ikan and sokutai should be strictly distinguished by the structure of the shozoku (costume) and occasions in which they are worn. One theory says that uniforms became lax in and after the mid Heian period and there were examples of 'ikan no sokutai' and 'noshi no sokutai' where ikan or noshi was used as an alternative of sokutai, and the particle 'no' was omitted from the former phrase, which was misused and then became established. However, when there are expressions such as 'noshi sokutai' in kuge diaries, they may mean 'there is someone in noshi or someone in sokutai' when you closely read them, and it is known there were shozoku called 'noshi sokutai' (composed of noshi no ho [round-necked robe]), ue no hakama (outer trousers), sekitai, shitagasane (long inner robe), etc. in the mid Heian period, so the theories above require rigorous verification.