Google Search, also referred to as Google Web Search or simply Google, is a web search engine developed by Google. It is the most used search engine on the World Wide Web across all platforms, with 92.62% market share as of June 2019, handling more than 5.4 billion searches each day. The order of search results returned by Google is based, in part, on a priority rank system called "PageRank". Google Search also provides many different options for customized search, using symbols to include, exclude, specify or require certain search behavior, and offers specialized interactive experiences, such as flight status and package tracking, weather forecasts, currency, unit and time conversions, word definitions, and more. The main purpose of Google Search is to search 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 in 1997 by Larry Page, Sergey Brin, and Scott Hassan. In June 2011, Google introduced "Google Voice Search" to search for spoken, rather than typed, words. 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 can be openly inquired via Google Trends and have been shown to correlate with flu outbreaks and unemployment levels, and provide the information faster than traditional reporting methods and surveys. As of mid-2016, Google's search engine has begun to rely on deep neural networks. Competitors of Google include Baidu and Soso.com in China; Naver.com and Daum.net in South Korea; Yandex in Russia; Seznam.cz in the Czech Republic; Qwant in France; Yahoo in Japan, Taiwan and the US, as well as Bing and DuckDuckGo. Some smaller search engines offer facilities not available with Google, e.g. not storing any private or tracking information. Within the U.S., as of July 2018, Bing handled 24.2 percent of all search queries. During the same period of time, Oath (formerly known as Yahoo) had a search market share of 11.5 percent. Market leader Google generated 63.2 percent of all core search queries in the U.S.