The Dow Jones Industrial Average measures the strength of American industry based upon how 30 large, publicly owned companies in the United States have traded in the stock markets. Companies come in and out the Dow periodically, according to whether their fortunes are waxing or waning.
When Apple joins the Dow Jones Industrial Average next week — replacing AT&T, which has been on the index since 1916 — the Dow will be at a historic high (assuming nothing catastrophic happens between now and then). But if Apple had joined the Dow in 2008, that value would be even more historic. It would have added more than 4,300 points to the Dow.