This post is typed verbatim from Etiquette Guide to Japan: Know the Rules... that Make the Difference, with authors' notes at the bottom.
From the earliest times, bowing (ojigi) has been the Japanese method of expressing greetings, saying farewell, paying respect, apologizing, showing humility, and indicating understanding and acceptance. The custom, which is common to many societies, no doubt had its origins in the animalistic behaviour of demonstrating submissiveness by dropping to the ground or lowering the head to avoid conflict with a stronger adversary. In particular, it likely became an institutionalized form of etiquette in religiously oriented societies where such behaviour was considered proper when in the presence of deities and their earthly representatives.
As with so many other behavioural traits, the Japanese took the practice of bowing much further than most people, developing it to a fine art and making it the only acceptable act in many different societal situations. During feudal times, failing to bow at the expected time or bowing improperly to a samurai or lord could result in a death sentence, sometimes carried out on the spot.
In earlier years, training in bowing began before babies could walk. In fact, their mothers and others would push their heads and trunks down repeatedly on the numerous daily occasions when bowing was the proper protocol. By the time children had reached school age, bowing was automatic, almost instinctive. The educational system and the maturing process honed bowing know-how, making it an integral part of the Japanese personality and character.
There are three specific types of bow: the light bow, the medium bow, and the deep bow. The latter, called sai-keirei, or "highest form of salutation", was commonly used during the shogunate period (1192-1868), but following the downfall of the last shogun in 1868, it was for the most part used only toward the emperor.
With the democratization of Japan following the end of World War II, the emperor renounced his divinity, and the use of the sai-keirei to pay obeisance to him gradually declined. Except for traditionalists- who are usually elderly- he is now treated like any other dignitary, i.e, when greeting him a medium bow has come to be entirely proper.
With the medium or formal bow the arms are extended downward with the hands resting on the legs above the knees. The body is then bent to about a forty-five-degree angle. The longer the bow is held the more meaning it has. In a normal situation it is only held for only two or three seconds.
With the light bow, the one most often used today, the body is bent to about a twenty-degree angle and the bow is held for only a second or so. The hands should be down at the sides when executing a light bow, but there are numerous occasions when this is impractical, such as when you are carrying something. The position of the hands has thus become more or less incidental, although it is polite to make an effort to bring them down to your sides.
Generally speaking, the medium bow is used when greeting dignitaries, when meeting those who are significantly senior to you and to whom you want to show a special degree of respect, and when expressing especially strong feelings of humility, sorrow, or apology to anyone.
If you are in a situation where you encounter the same dignitaries or highly placed seniors several times in one day, you should greet them with a medium bow the first time you meet them that day and a light bow thereafter.
The influence of the bow in Japanese society is so powerful that foreign residents who are studying the language and associate frequently with Japanese are susceptible to picking up the custom by osmosis. I still sometimes catch myself bowing when I am talking to a Japanese on the telephone!
Young, urban Japanese mothers virtually gave up on the custom of teaching their children from infancy the custom of bowing. Nowadays, children are required to bow in school and on numerous other social occasions, but the practice is not being instilled into their reflexes or psyche, as it had been in the past. Young people entering the work force from the 1980s, especially those entering the retail service industry, have had to be taught to bow as part of their company training.
The bow remains a vital part of daily life and work in Japan, and it is not likely to disappear within the forseeable future even though the younger generations are assuming a much more casual attitude toward it. There is, in fact, a pronounced tendency among Japanese to gradually revert to traditional attitudes and forms of behaviour as they age. They find many of the old customs more satisfying and fulfilling than practices copied from the West.
The foreign visitor does not have to be overly concerned about when and how to bow when dealing with Japanese. Once again, Japanese regard the bow as a custom of their own particular culture and tend to believe that foreigners cannot be expected to do it properly- although to not make any attempt at all may still be regarded as impolite or arrogant.
The best rule to follow in a one-on-one situation is to bow when the Japanese do and to be wary of bowing too low or for too long when the occasion does not call for it. In other words, it is better to err on the conservative side to avoid being considered insincere or foolish.
Customers at department stores and other public places are not expected to return the repeated bows of store employees. But the bow of a receptionist in a company lobby should be acknowledged with a slight nodding of the head. A casual nod of the head is also all that is usually called for in hotels and in restaurants, places where the staff regularly bows to guests.
Keep in mind that deep, long bows are reserved for occasions when one demonstrates extraordinary appreciation, respect, humility, or sorrow. Again, older people, especially longtime friends who do not see each other often, will typically bow deep and long as a way of expressing deeply felt emotions. When bows involve old friends, they are the Japanese equivalent of a warm embrace.
Previously: Let's Talk (about Dowa)
Next: Meiyo, Honour