The Thai greeting referred to as the wai (Thai: ไหว้, pronounced [wâːj]) consists of a slight bow, with the palms pressed together in a prayer-like fashion. It has its origin in the Indic Añjali Mudrā, like the Indian namasté and the Cambodian sampeah. The higher the hands are held in relation to the face and the lower the bow, the more respect or reverence the giver of the wai is showing. The wai is traditionally observed upon entering formally a house. After the visit is over, the visitor asks for permission to leave and repeats the salutation made upon entering. The wai is also common as a way to express gratitude or to apologise.
The word often spoken with the wai as a greeting or farewell is sawatdi (RTGS for สวัสดี, pronounced [sàwàtdiː], sometimes romanized as sawasdee). The word sawatdi is usually followed by kha when spoken by a female and by khrap when spoken by a male person. This word was coined in the mid-1930s by Phraya Upakit Silapasan of Chulalongkorn University. This word, derived from the Sanskrit svasti (meaning "well-being"), had previously been used in Thai only as a formulaic opening to inscriptions. The strongly nationalist government of Plaek Pibulsonggram in the early 1940s promoted the use of the word sawasdee amongst the government bureaucracy as well as the wider populace as part of a wider set of cultural edicts to modernise Thailand.
Waiing remains to this day an extremely important part of social behavior among Thais, who are very sensitive to their self-perceived standing in society. As a rule of thumb for foreign tourists and other visitors unaccustomed to the intricacies of Thai language and culture, it is unwise to initiate a wai exchange with someone who is younger. However, one should always return a wai that is offered as a genuine sign of respect. Corporate wais, such as those performed by convenience store cashiers, can generally be 'returned' with a smile.

If one is waiied while carrying goods, or for any other reason that makes the physical act of returning a wai difficult, it is still important to recognize the show of respect and make a physical effort to wai back as best as possible under the circumstances

The wai is the common form of greeting and adheres to strict rules of protocol.
Raising both hands, palms joined with the fingers pointing upwards as if in prayer, lightly touching the body somewhere between the chest and the forehead, is the standard form.
The wai is both a sign of respect as well as a greeting. Respect and courtesy are demonstrated by the height at which the hands are held and how low the head comes down to meet the thumbs of both hands.
The wai may be made while sitting, walking, or standing.
The person who is junior in age or status is the first one to offer the wai.
The senior person returns the wai, generally with their hands raised to somewhere around their chest.
If a junior person is standing and wants to wai a senior person who is seated, the junior person will stoop or bow their head while making the wai.
If there is a great social distance between two people, the wai will not be returned.