Since 1978, when we published our first list of 200 firms, and 1983, when we expanded the list to 250, The National Law Journal‘s annual law firm survey has identified the largest legal players in the United States — ranking them by headcount and providing detail about their locations, partners, associates and other attorneys.

This year, we launched our biggest-ever expansion of the survey, adding 100 additional firms to our list, ranked from No. 251 to No. 350. The new list provides a closer look at the growing midsize law firm market and includes a number of regional players outside the big cities that dominate our traditional NLJ 250 survey.

In fact, 59 different markets are represented on the list — from Billings, Mont. — home to the 124-lawyer Crowley Fleck — to Columbia, S.C., headquarters for the 113-lawyer McNair Law Firm.

That’s not to say big cities don’t appear on the 350. Several firms are concentrated in traditionally large legal markets: Chicago is home to the most firms (eight), followed by New York (six), Los Angeles and Cleveland (five each). It’s just that the geographic differences are far more pronounced than among the upper tiers of our survey.

As for headcount, more than 13,000 lawyers work at the 100 firms on our expanded list. Firms on the NLJ 350 range in size from 112 to 160 lawyers and the average firm size is 133. By way of comparison, among firms ranked No. 250 or higher the average size is 505 lawyers.

But the numbers among the top 250 are increasingly skewed by the ever more massive size of firms at the very top of the ranking. Divide the NLJ 350 into seven 50-firm tiers, and the separation among the top 50 and the rest of firms on the survey become clearer. Firms in tier 1 (nos. 1-50) are growing at a faster clip than their NLJ 350 peers. In 2002, firms in the top tier averaged 954 lawyers, about 97 percent more attorneys than the firms in tier 2 (nos. 51-100). Today, tier 1 firms employ 1,216 lawyers on ­average, 123 percent more than tier 2. Among other tiers the change has been far less pronounced. For instance, the difference between tiers 2 and 3 was 50 percent in 2002 and 56 percent in 2012.

The NLJ survey has, for more than three decades, defined the borders of Big Law. The list of 100 firms in this special report is a recognition that the large firm universe has expanded. We still believe a 100-lawyer law firm is a big firm. But the differences between the firms at the top of our survey and the bottom have never been more pronounced. How much will the gap widen in the years to come?

— David L. Brown, editor in chief