Two tech companies and a consulting firm make an unusual trio, but according to the employer review site Glassdoor they share one key distinction.

Google, Facebook, and Bain & Company are the three best workplaces in the US.

Since 2009, when Glassdoor began collecting data for its Best Places to Work ranking, the three companies have consistently made the list, and lately they’ve stayed ins the top 10. Bain & Co. was ranked the best workplace of 2017, Google the best of 2015, and Facebook the best of 2014.

There are a number of ways to explain what makes the companies great. Here are the biggest factors.

SEE ALSO: What it’s like to work at Bain & Company, the best workplace of 2017

They support open cultures.

Great workplaces demonstrate transparency between leadership and fellow staff.

At Google, cofounders Larry Page and Sergey Brin routinely hold company-wide meetings on Friday, called TGIF, where employees can ask any question of the company they want.

“It’s truly fair game to ask anything, no matter how controversial, and frequently the executives will be responsive,” one employee wrote on Glassdoor.

Offices are designed to maximize productivity.

To help people reach peak productivity, the best companies provide a range of rooms, nooks, and workspaces.

At Bain and Co., employees can steal away into one-person quiet spaces, camp out in private, high-back chairs, or mingle in the open office with others.

Facebook and Google make similar design choices in their offices, outfitting them with large and small conference rooms and breakout areas for quick, private chats.

They pay people well.

Still, it doesn’t hurt to make a lot of money.

The typical consultant and Bain and Co. makes $140,000 plus $30,000 in bonuses, stock options, and profit sharing. Google employees make about the same.

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