'A big and fantastic moment.' Last year, I wrote about why Wall Street's big banks had never had a female CEO. At the end of the story, I asked which institution would be the first to break that unfortunate streak. I got my answer yesterday, when Citigroup announced that Jane Fraser will become CEO when Michael Corbat retires in February.

As my colleague Maria Aspan reported yesterday, the appointment was an expected transition. Corbat named Fraser as his No. 2, tagging her as his heir apparent, last October. Still, Fraser's new role is unprecedented.

Wall Street had corporate America's most stubborn glass ceiling. Several women had come close to breaking it before the 2008 recession—Morgan Stanley's Zoe Cruz, Lehman Brothers' Erin Callan, Citi's Sallie Krawcheck—but the ceiling stayed intact, even amid the fallout of the Great Recession, even as banks' workforces diversified, even through the height of the #MeToo movement, even in the most analytical of industries that prizes performance above all else.

So women in the industry reacted with downright glee yesterday when Fraser finally smashed through.

"Congratulations to Citi for naming Jane Fraser as its next CEO! Great news for the company and for women everywhere! A big and fantastic moment," Cathy Bessant, Bank of America's chief operations and technology officer, tweeted.

"CEO jobs don’t come open very often," said Betsy Duke, former Wells Fargo chair. "In the past the reason given for not choosing a woman was that none were ready. Jane is the first of many who are ready to step into these jobs now."

Of course, Fraser's promotion isn't necessarily indicative of systemic change in the industry. In fact, it shows just how much of an outlier Citi is now. Citi's board is already the most gender-diverse among S&P 500 banks. And with Fraser's new appointment, it will become the only bank board at gender parity, according to Bloomberg data. On the operating teams that oversee the six biggest U.S. banks, collectively there are still two men for every woman.

When I asked women last year about the factors keeping women from ascending to the top of banking's corporate ladder, not one cited the kind of blatant, sleazy sexism that once plagued trading floors. The bias, they said, was far more subtle than that, from “microaggressions” to “over-mentoring” and “under-sponsoring.” "It’s the small things, like men going out for a drink or men taking a business trip together," Donna Milrod, head of State Street’s global clients division, told me last year. Those small things that disadvantage women "[become] emphasized over time."

The small stuff won't disappear in February. But for the first time, someone leading a big bank might know what the small stuff feels like.