The Monarchy:
Thai people have a deep, traditional reverence for the Royal Family, and a visitor should be careful to show respect for the King, the Queen and the Royal Family.
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Religion:

Visitors should dress neatly in all religious shrines. They should never go topless, or in shorts, hot pants or other unsuitable attire. It is acceptable to wear shoes when walking around the compound of a Buddhist temple, but not inside the chapel where the principal Buddha image is kept.

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Every Buddha image, large or small, ruined or not, is regarded as a sacred object. Never climb onto one to take a photograph or do anything that might indicate a lack of respect. Buddhist monks are forbidden to touch or be touched by a woman, or to accept anything from the hand of one. If a woman has to give anything to a monk, she first hands it to a man, who then presents it, or puts it down and allows the monk to pick it up
Social Norms:

Thais don't normally shake hands when they greet one another, but instead press the palms together in a prayer-like gesture called a 'wai' . Generally, a younger person wais an elder, who returns it. It is however far more complex than this and for tourists a simple smile is acceptable when someone wais you. Thais regard the head as the highest part of the body, literally and figuratively. Therefore, avoid touching people on the head and try not to point your feet at people or an object. It is considered very rude.Shoes should be removed when entering a private Thai home. Public displays of affection between men and women are frowned upon.

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Special Advice:

Beware of unauthorised people who offer their services as guides. For all tourist information, contact the Tourism Authority of Thailand, Tel : 1672. For information about Bangkok, contact the Bangkok Metropolitan Tourist Bureau, Tel : 0 2225 7612-4. Observe all normal precautions as regards to personal safety, as well as the safety of your belongings. Walking alone on quiet streets or deserted areas is not recommended. Be sure that all your valuables-money, jewellery, and airline tickets are properly protected from loss.

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Do not get yourself involved with drugs. Penalties for drug offences are very severe in Thailand.