Wade–Giles
One well-known feature of Wade-Giles is that it uses the apostrophe to mark aspirated consonants, 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 voiced stop 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.