Google Search, commonly referred to as Google Web Search or just Google, is a web search engine owned by Google Inc. It is the most-used search engine on the World Wide Web, handling more than three billion searches each day. The order of search on Google’s search-results pages is based, in part, on a priority rank called a “PageRank”. Google Search provides many different options for customized search, using Boolean operators such as: exclusion (“-xx”), alternatives (“xx OR yy OR zz”), and wildcards (“Winston * Churchill” returns “Winston Churchill”, “Winston Spencer Churchill”, etc.) The same and other options can be specified in a different way on an Advanced Search page. The main purpose of Google Search is to hunt for text in publicly accessible documents offered by web servers, as opposed to other data, such as images or data contained in databases. It was originally developed by Larry Page and Sergey Brin in 1997. Google Search provides several features beyond searching for words. These include synonyms, weather forecasts, time zones, stock quotes, maps, earthquake data, movie showtimes, airports, home listings, and sports scores. There are special features for numbers, dates, and some specific forms, including ranges, prices, temperatures, money and measurement unit conversions, calculations, package tracking, patents, area codes, and language translation. In June 2011 Google introduced “Google Voice Search” to search for a spoken, rather than typed, word. In May 2012 Google introduced a Knowledge Graph semantic search feature in the U.S. Analysis of the frequency of search terms may indicate economic, social and health trends. Data about the frequency of use of search terms on Google have been shown to correlate with flu outbreaks and unemployment levels, and provide the information faster than traditional reporting methods and surveys. Competitors of Google include Baidu and Soso.com in China; Naver.com and Daum Communications in South Korea; Yandex in Russia; Seznam.cz in the Czech Republic; Yahoo! in Japan, Taiwan and the United States, as well as Bing and DuckDuckGo. Some smaller search engines offer facilities not available with Google, e.g. not storing any tracking information.