Tokyo Stock Price Index, commonly known as TOPIX, along with the Nikkei 225, is an important stock market index for the Tokyo Stock Exchange in Japan, tracking all domestic companies of the exchange's Prime market division. It is calculated and published by the TSE. As of 1 February 2011, there were 1,669 companies listed on the First Section of the TSE, and the market value for the index was ¥197.4 trillion. The index transitioned from a system where a company's weighting is based on the total number of shares outstanding to a weighting based on the number of shares available for trading. This transition took place in three phases starting in October 2005 and was completed in June 2006. Although the change is a technicality, it had a significant effect on the weighting of many companies in the index, because many companies in Japan hold a significant number of shares of their business partners as a part of intricate business alliances, and such shares are no longer included in calculating the weight of companies in the index. Wikipedia