Bill Gates, in town for the U.N. climate meetings recently, posted a selfie with Mike Bloomberg.

What caught my eye was the lanyard around Mike’s neck.

He was wearing his corporate badge even though he wasn’t anywhere near the Bloomberg office. (The photo was taken near 43rd Street and 6th Avenue.)

Badges are a big deal at Bloomberg and a big deal for Mike.

Since the 9/11 attacks, lots of companies require badges, but most use them for security and few require employees to wear them all day.

I suspect few CEOs have given badges as much thought as Mike, who has leveraged them as a communication tool and cultural identity marker as much as for security.

Everyone at Bloomberg wears a badge. Full-time employees get badges with orange borders. Badges for contractors are green. Visitors get paper badges. Even the security dogs wear badges with photos and names. (Fun fact: The dogs are contractors and therefore sport green.)

London Mayor Sadiq Khan and architect Lord Norman Foster visited the U.K. office and were awarded orange employee badges as a kind of honorific.

I know of only one person who didn’t wear one. In 2018, Oprah visited the office and wasn’t badged. It was so notable that a reporter tweeted about it.

The badges were first created in the mid-1990s to swipe into the office.

One key reason at the time was because clients were calling into the switchboard and the operators didn’t know who was in the building. Badges reduced the use of overhead paging.

Mike took the additional step of having employees wear them. It helped him (and everyone else) identify one another.

The badges solved one problem, but created another: employees would forget them.

It created a conundrum for security. Should they let the people in or not?

By early 1998 the issue had become acute, prompting Mike to send out a company-wide email that said as many as 65 employees a day were forgetting their badges.

He wrote: “To assist our receptionists, I have volunteered to come down to the lobby whenever this happens in the future and escort those of you without badges into the building.”

He added that he would be traveling in Europe for three days of the following week so anyone who forgot their badge then would be out of luck.

People stopped forgetting.

The policy was later relaxed. A companywide email sent that October explained that anyone who forgot their badge would be allowed two replacements each year.

If you forgot your badge for a third time, however, your manager and HR would be notified and you would lose a vacation day.

Mike left for a dozen years to serve as mayor of New York City. When he returned in 2014, he had the badges redesigned with the first name much larger, so they were even easier for everyone to read.

When you are building a winning culture, you need to know who you are building it with.

(Part of a series of management lessons learned from three decades at Bloomberg)