Shares of Blackstone Inc. BX gained 4.1% in the after-market trading on Friday after S&P Dow Jones Indices said that the private equity firm will join the S&P 500 index. In a press release, S&P Dow Jones Indices said that as part of the quarterly rebalancing, BX will replace Lincoln National Corp. prior to the start of trading on Sep 18. Notably, Blackstone has a dual share structure with unequal voting rights. After the S&P Dow Jones gave a decision in April to no longer follow a 2017 rule that barred corporations with multiple share classes from index membership, BX was expected to be added to the S&P 500. Another company to join the S&P 500 index on the same date is Airbnb, Inc. ABNB. Following the announcement by the S&P Dow Jones Indices, shares of ABNB rallied 5% in the after-market trading.