The hyphen is only used when writing translated foreign names with hyphens. This section includes rules regarding hyphens, commas, parentheses and quotations. Apostrophes (as well as hyphens and tone marks) are omitted on Chinese passports. Special characters, such as hyphens and exclamation marks, are not permitted. These numbers are six digits long, formatted into three pairs which are separated by hyphens. Frequently the subdomain matches the linked phrase, with spaces replaced by underscores or hyphens. The use of hyphens is called hyphenation. Otherwise the two names are simply both given, joined by hyphens or slashes. They are up to 31 characters long and may include letters, digits, hyphens and underscores. Furthermore, leading zeroes are used in addition to hyphens between the characters. Hyphens are often used in English compound words. A few theories have been offered for the hyphens and capitalization. Words in smikhut are often combined with hyphens. Each language tag is composed of one or more "subtags" separated by hyphens (-). The hyphens and lack of projection lines indicate prosodic dependencies.