I am quite a fan of traditional Thai clothing, chut thai (ชุดไทย), which literally means “Thai outfit”. Generally, the items are made of colorful silk or hand-woven cotton and consists of several elements. For women, the parts are the pha sin (ผ้าซิ่น) in northern regions of Thailand which is a handmade traditional tube skirt, the pattern of which can indicate which region the wearer is from; the pha nung (ผ้านุ่ง) or chong kraben (โจงกระเบน), a unisex, lower body, wraparound cloth which is more of a pant than a skirt with the wearer wrapping a rectangular piece of cloth around his [or her] waist, the edge of cloth is then passed between the legs and tucked in at the wearer’s lower back; the phaa biang (ผ้าเบี่ยง), a shawl-like breast wrapper; and a blouse. Men usually wear a set known as the Raj pattern (ราชปะแตน) which refers either to a Thai men’s costume consisting of a white Nehru-style jacket with five buttons, a chong kraben, knee-length socks and dress shoes, or to the specific form of the jacket itself. There are several variations on this.
There is also what is known as the formal Thai national costume, chut thai phra ratcha niyom (ชุดไทยพระราชนิยม), literally ‘Thai dress of royal endorsement’. This includes several sets of clothing, or chut thai, designed for use as national costume on formal occasions. Although described and intended for use as national costume, they are of relatively modern origins, having been conceived in the second half of the twentieth century.

For an overview with illustrations, have a look at this posting on Asia Highlights. Any Thai festival or dance involving traditional culture are a feast for the eyes with a wide variety of interesting clothing on display.
As far as clothing worn by foreigners, most types of attire are tolerated in areas frequented by tourists. It is a good idea, however, to cover up when visiting temples and shrines. Those wearing sleeveless tops, short skirts, shorts, and flip flops may be denied entrance. However, you will see women – particularly in Bangkok and Phuket – dressed in short skirts, heels and backless tops, and think, “That’s a bit dressy just to go to the mall”!
This may appear to you as a green light to dress liberally, but in actual fact, as a foreign woman, you are expected to exercise modesty – if you want to be perceived in a good light.
It’s a real double standard.
The perception of Western women is one of a highly sexed bunch who like to let it all hang out; a notion borne out of Western movies and different cultural norms, I assume.
It may feel hypocritical when you see a Thai woman walk past not wearing a bra underneath her vest top with her nipples poking through and cleavage hanging low, but that’s just one of those “Thainess” things you’re going to have to get past I’m afraid. Paradoxically, you might notice that many of these same women will wear shorts under their skirts as an extra layer of modesty. For the same reason, it is common for Thai men to first put on a pair of tight underwear and then to wear a pair of boxer shorts on top of them before they put on their jeans or trousers.

There is actually a law in Thailand that states that it is illegal for men to drive a motor vehicle topless. It doesn’t mention any prohibition against women doing the same but I suppose that is just common sense. Certainly, many a hot and bothered foreigner has fallen foul of this law believing the country’s relaxed attitude to what happens on beaches extends to the rest of Thailand.
It is not unusual to encounter signs prohibiting women from entering highly sacred places, such as temple libraries. Women who wish to worship do so outside the buildings.

One must also take care to dress appropriately when visiting schools and government offices. Many of the latter will post images of what to wear and what not to wear. In addition to these prohibitions, additional ones may be put into place with very little notice. An example of this was the mourning period when the late HRH King Bhumibhol Adulyadej died in October 2016. Black or greys was the order of the day; at the beginning, there was a shortage of black clothing and so enterprising individuals set up sidewalk stalls to dye one’s white clothing to black. Price restrictions had to be implemented so that shops selling black clothing could not make a profit on them. However, the government announced that tourists were not required to wear black on the beaches of Thailand.
