In the early days of mobile phones, ringtones were limited to a few simple sounds. The most popular ringtones were the "ring-ring" sound of a traditional phone, or a simple beep. As mobile phones became more popular, ringtones became more elaborate. Today, there are millions of ringtones available for download, ranging from simple melodies to complex songs. Ringtone popularity varies from country to country. In some countries, like the United States, ringtones are primarily used to identify the phone owner to their friends and family. In other countries, like Japan, ringtones are used as a form of self-expression, with people choosing ringtones that reflect their personality or taste in music. There are two main types of ringtones: monophonic and polyphonic. Monophonic ringtones are simple tones that can only play one note at a time. Polyphonic ringtones are more complex, and can play multiple notes at the same time. Polyphonic ringtones are more popular than monophonic ringtones, because they sound more like real music. Most phones today come with a built-in ringtone player, and many phones can also download ringtones from the Internet. There are also a number of companies that sell ringtones, either as individual files or as part of a monthly subscription.