CMC can either be synchronous (exchanges take place in 'real time') or asynchronous (messages are posted up at any time, and read and responded to by other users also at times which suit them; in other words, users do not have to be online at the same time, as they do with synchronous exchanges). Email, mailing lists, Usenet and computer conferencing are all asynchronous, while IRC, Internet telephony and videoconferencing all take place synchronously. All of these types of CMC are now available through the Web i.e. through a standard Web browser.