S&P 501

When Google effectively splits its stock on Wednesday, S&P Dow Jones Indices will do something unprecedented: It will keep both the old Google shares and the new ones in the S&P 500. That means the S&P 500 will technically have 501 components, though it will still have only 500 companies.

In the past, S&P Dow Jones Indices, the company that runs the S&P 500, has not kept the additional shares that more than 40 S&P 500 companies offer in the index.

In fact, in a February press release, the company announced that it would switch Google from the Class A shares (which will trade under the ticker "GOOGL") to the class C shares (which will trade under the ticker "GOOG," and are likely to be more liquid).

But in a March press release, it revised that decision, and said both the Class A and Class C shares will be included in the S&P 500 (as well as in the S&P 100).