Although all Thais have a first name and a family name, everyone is addressed by their first name – even when meeting strangers – prefixed by the title "Khun" (Mr/Ms but literal translation is 'you'); no one is ever addressed as Khun Surname, and even the phone book lists people by their given name. In Thailand you will often be addressed in an Anglicized version of this convention, as "Mr Paul" or "Miss Lucy" for example. Bear in mind though, that when a man is introduced to you as Khun Pirom, his wife will definitely not be Khun Pirom as well (that would be like calling them, for instance, "Mr and Mrs Paul"). Among friends and relatives, Phii ("older brother/sister") is often used instead of Khun when addressing older familiars (though as a tourist you're on surer ground with Khun), and Nong ("younger brother/sister") is used for younger ones.
Many Thai first names come from ancient Sanskrit and have an auspicious meaning; for example, Boon means good deeds, Siri means glory and Thawee means to increase. However, Thais of all ages are commonly known by the nickname given them soon after birth rather than by their official first name. This tradition arises out of a deep-rooted superstition that once a child has been officially named the spirits will begin to take an unhealthy interest in them, so a nickname is used instead to confuse the spirits. Common nicknames – which often bear no resemblance to the adult's personality or physique – include Yai (Big), Oun (Fat) and Muu (Pig); Lek or Noi (Little), Nok (Bird), Noo (Mouse) and Kung (Shrimp); Neung (Number One/Eldest), Sawng (Number Two), Saam (Number Three); and English nicknames like Apple, Joy or even Pepsi.
Family names were only introduced in 1913 (by Rama Vl, who invented many of the aristocracy's surnames himself), and are used only in very formal situations, always in conjunction with the first name. It's quite usual for good friends never to know each other's surname. Ethnic Thais generally have short surnames like Somboon or Srisai, while the long, convoluted family names – such as Sonthanasumpun – usually indicate Chinese origin, not because they are phonetically Chinese but because many Chinese immigrants have chosen to adopt new Thai surnames and Thai law states that every newly created surname must be unique. Thus anyone who wants to change their surname must submit a shortlist of five unique Thai names – each to a maximum length of ten Thai characters – to be checked against a database of existing names. As more and more names are taken, Chinese family names get increasingly unwieldy, and more easily distinguishable from the pithy old Thai names.