No one could have predicted that an obscure legal structure designed to be used by Swiss sports clubs and other nonprofits would end up fundamentally reshaping the legal industry.

The Swiss verein, which allows firms to combine more easily and retain their existing forms, has been adopted in almost every major cross-border law firm tie-up of the past decade, including those that created Dentons, DLA Piper, Hogan Lovells, King & Wood Mallesons, Norton Rose Fulbright and Squire Patton Boggs. (Baker McKenzie and Littler Mendelson are also vereins.) Vereins now account for five of the world’s 15 largest law firms by revenue, according to The American Lawyer’s Global 100 survey—including three of the top six.