Wade-Giles was the main system of transliteration in the English-speaking world for much of the 20th century. It is used in several standard reference books. It is also used in all books about China published before 1979. It replaced the Nanjing-based romanization systems that was common until the late 19th century. Wade-Giles has been replaced by the Pinyin system today. It still remains in use in Taiwan (Republic of China).
One well-known feature of Wade-Giles is that it uses the apostrophe to mark aspiratedconsonants, or breathy consonants. For example, ping in Pinyin would be written as p'ing in Wade-Giles, but bing in Pinyin would be written as ping. Sounds are written this way because Chinese does not have voicedstop consonants, fricatives, or affricates like in English. Chinese only distinguishes between aspirated and unaspirated sounds. Therefore, an apostrophe is used instead of using a different letter.
However, the apostrophe was not well understood by people who do not know the Wade-Giles spelling, so many people who read the apostrophe ignored the apostrophe when reading or copying Chinese words down. This had lead many non-Chinese to say Chinese words incorrectly, including Tao, tai chi, and kung fu. Wade-Giles spellings where the apostrophe is completely ignored are called bastardized Wade-Giles.