The current bull market in stocks has been going on for more than 10 years, and many companies have produced amazing recoveries in their share prices following the 2008-2009 recession. When you take the total number of shares companies have outstanding and multiply them by their share price, you get what's called their market capitalization. A couple of big-name tech stocks have seen their market caps jump above the $1 trillion mark -- and keep heading higher from there.

Apple (NASDAQ:AAPL) was the first U.S. company to reach the illustrious milestone in the summer of 2018, and Amazon.com was quick to follow suit. Yet neither company stayed there long, and this year, Microsoft (NASDAQ:MSFT) was the first to find the $1 trillion mark before Apple rejoined it.

As you can see below, Apple and Microsoft have continued to vie for the top spot, while Amazon and fellow internet giant Alphabet remain well behind.

Chart showing market capitalizations of the top four U.S. stocks

Data source: Yahoo! Finance. Chart by author.

The reasons for Apple's and Microsoft's respective ascendancies are related but a bit different. For Apple, the continued popularity of its iPhones has been instrumental in producing revenue, but investors have started to look at other elements of its business. In particular, offerings like the Apple TV+ streaming video service and the company's App Store have the potential to produce recurring revenue that will bolster overall growth for years to come. Shareholders have high hopes for those parts of the business, seeing services as a way to offset sluggish performance in selling hardware.

Meanwhile, Microsoft's success has come largely from embracing cloud computing. For years, the software giant resisted the cloud trend, but when CEO Satya Nadella joined the company, he moved Microsoft to start offering its software products on a subscription basis. That strategy has proven to be extremely profitable, and after seeing its stock languish for most of the 2000s, Microsoft has finally pushed itself to new heights.

With both Microsoft and Apple above the $1.1 trillion mark, the big question is how high they can climb -- and whether others will join them. The bull market will eventually come to an end, but with major benchmarks setting new highs again, few investors seem to think that end is coming soon.