In less than twenty years, mobile telephones have gone from being rare and expensive pieces of equipment used by businesses to a pervasive low-cost personal item. In many affluent countries, mobile phones now outnumber land-line telephones, with most adults and many children now owning mobile phones. It is not uncommon for young adults to own simply a cell phone instead of a land-line for their residence, even in the U.S. where mobile phone use is less prevalent than other industrialized countries. Mobile phone penetration is increasing around the world; this is particularly true of developing countries, where there is little existing fixed-line infrastructure. With high levels of mobile telephone penetration, a mobile phone culture has evolved, where the mobile phone becomes a key social tool, and people rely on their mobile phone address book to keep in touch with their friends. Many people keep in touch using SMS, and a whole culture of "texting" has developed from this.