Tesla, one of the best performing US stocks this year, just suffered its worst trading day since it went public 10 years ago. Shares of Tesla\n \n (TSLA) lost 21% of their value Tuesday after Standard & Poor’s declined to add it to its index of 500 major US stocks. Being added to the index would have required portfolio managers who mirror the index to buy additional shares. Part of the rise in Tesla\n \n (TSLA) shares in recent months was on anticipation that would happen. But while S&P Indexes announced late Friday that it was adding Etsy, an online marketplace for crafters; Teradyne\n \n (TER), a company specializing in industrial automation and robotics; and Catalen\n \n (CTLT)t, which develops pharmaceuticals, to the index, the absence of Tesla was a big disappointment for investors, prompting the sell-off. The index requires companies to have a certain market cap and history of profitability in order to be added. Tesla easily clears the market cap threshold, being the most valuable automaker in the world. But its profitability is somewhat new. It posted its first annual operating profit in 2019. Tuesday’s 21% tumble edged out the company’s 19% slide in January 2012, less than two years after its initial public offering. The company’s third and fourth largest one-day drops came when it lost nearly 19% on March 16, and 17% on February 5 of this year. The stock rebounded nicely from each of those sell-offs. Still, even with the 21% decline Tuesday, Tesla shares have nearly quadrupled in value so far this year, rising 295%. The stock is down 34% since reaching a record high close on Aug. 31. Tech stocks overall have been battered during this period, as the tech-heavy Nasdaq is down 10% since its own record high close on Sept. 2. Fortunately for Tesla, it announced early Tuesday that it completed the sale of $5 billion worth of its shares last week before Tuesday’s drop.