The changing face of the Dow: See the names that have come and gone on the path to 40,000

The Dow Jones Industrial Average reached a key milestone on Thursday, the latest in a more than 100-year ascent that's involved an everchanging group of stocks.

The closely followed average on Thursday broke above the 40,000 marker for the first time in its history. But the index today looks far different than it did a century ago — or even just a few decades prior.

The average was created by Charles Dow in 1896 with just 12 industrial stocks. Paired with the Dow Jones Transportation Average, the two were collectively meant to offer a gauge for the broader economy. After eight stocks were added in 1916, followed by another 10 in 1928, the Dow has stood for nearly a century at 30 members.

To reflect the country's industrial presence, companies have been added and removed over time as the economy has evolved. Take technology, for example. There are now seven stocks from the sector in the index, reflecting its growing prominence in the equity market and corporate world.

A committee governs stock selection for the esteemed group of blue-chip stocks, according to S&P Dow Jones Indices, the S&P Global-owned company that operates the benchmark. Though it has no defined rules, the group typically analyzes a firm's reputation, growth over a long period and relevance to investors. A sector's representation in the Dow compared with its place in the broader market can also play a role.

The sole caveat: No utility or transportation stocks are included, given the existence of the Dow Jones Utility Average and Transportation Average.

With the Dow hitting its next big milestone, CNBC looked back at which companies comprised the index at previous important moments.

It may be hard to imagine given the recent achievement, but the Dow traded below