ToASCII leaves unchanged any ASCII label, but will fail if the label is unsuitable for the Domain Name System. If given a label containing at least one non-ASCII character, ToASCII will apply the Nameprep algorithm, which converts the label to lowercase and performs other normalization, and will then translate the result to ASCII using Punycode before prepending the four-character string "xn--". This four-character string is called the ASCII Compatible Encoding ("ACE") prefix, and is used to distinguish Punycode encoded labels from ordinary ASCII labels. The ToASCII algorithm can fail in several ways; for example, the final string could exceed the 63-character limit of a DNS label. A label for which ToASCII fails cannot be used in an internationalized domain name.