As merger interest in the legal industry runs rampant, several Big Law firms are in merger talks or in the process of integrating from a merger. But an untold number of law firms have left merger discussions in recent years, halted by differences over culture, financials or leadership.

A look back at one of Big Law’s most famous mergers—the combination of Boston-founded Hale and Dorr with Washington, D.C.-founded Wilmer, Cutler & Pickering in 2004—provides some clues to firms currently in talks about navigating the merger process and what worked. With the merged firm, informally known as WilmerHale, now in its 20th anniversary, firm leaders spoke with about some culture clashes at the beginning but also how and why the firms successfully integrated.