It's a derisive, almost dismissive term: "National League stuff." It refers to pitchers who, sure, get by in the Senior Circuit, but don't have what it takes when the competition gets tougher, when the lineups feature nine hitters instead of eight.

And there seems to be something to it, or at least to the broader concept.

From the All-Star Game to Interleague Play -- and to a lesser extent, even the World Series -- it's evident that a gap has opened between the leagues. The sledding is tougher in the American League, where the Red Sox, Yankees, Angels, White Sox and now the Rays set a fiercely high standard.

There's evidence on an individual level, too. In recent years, starting pitchers moving from the NL to the AL have seen a marked dip in their performance levels. It has to be a warning light to any American League club pondering Ben Sheets, Randy Johnson or Derek Lowe -- never mind lesser lights like Randy Wolf, Oliver Perez or Braden Looper.

Some of those guys, of course, can't be accused of "NL stuff," since they've succeeded in both leagues. Lowe parlayed success in Boston into a big contract with the Dodgers. Johnson made his name with the Mariners, though his tenure with the Yankees wasn't exactly all sunshine and light.

But the fact is, any American League club considering a National League pitcher will need to include some sort of exchange rate, perhaps akin to how you view a hitter's stats when he leaves Coors Field. And it isn't just that extra hitter in the lineup. They fare worse relative to their new peers, not just their opponents, and that's the truly striking thing.

The numbers are stark. Eight times since the start of the 2000 season, a pitcher has made 30 starts in the National League followed by 30 in the AL the next year. Five other times, a pitcher has missed it by one start in either the first or second year.

That's a total of 13 cases, not counting pitchers who changed leagues after a year with few starts, or got hurt in their first year in the new league. Admittedly, it's not enough to be scientific, but it's not just one or two guys, either.

Of those 13 transitions, only once did the pitcher have a clearly better year in the AL, and only twice did they even hold steady. The other 10 all experienced a dropoff in performance, ranging from noticeable to precipitous.

As a group, the 13 pitchers averaged an ERA+ (a measure of ERA relative to the league average) of 116 in their NL seasons, dropping to 99 in their AL seasons. They went from quite a bit above average to a hair below. Their average strikeout-to-walk ratio fell from 3.11 to 2.46, their WHIP climbed from 1.24 to 1.37, and their innings per start fell from 6.31 to 6.08.

Collectively, it's like the transition from Josh Beckett (career ERA+ of 116, WHIP of 1.22) to Vicente Padilla (100, 1.39).

The list includes at least one Hall of Famer, Randy Johnson. It includes a couple of guys who will at least get some votes in Beckett and David Wells. It includes numerous All-Stars, quality pitchers like Javier Vazquez, Kevin Appier and Matt Clement. Other names on the list: Miguel Batista (twice), Mark Redman (twice) and Kris Benson.

The only pitcher who improved? Well, he comes with a bit of an asterisk: John Thomson, who in leaving the NL also escaped Coors Field. Thomson's ERA+ improved from 96 to 104, his K/BB rate went up by about 14 percent and he added about one-third of an inning every two starts. Wells was about as effective in either league, and Batista experienced only a slight dropoff the second time around.

Everybody else fell off.

And here's the thing to note: it's not just the hitting environment that changed. That's what using ERA+, rather than ERA, adjusts for. Beckett's 118 ERA+ in his last year as a Marlin meant he was 18 percent better than other NL starters in 2005. His 95 in his first year with the Red Sox meant he was five percent worse than other AL starters in '06.

That's what should truly give AL general managers pause when they look at NL pitchers. It's long been accepted that there's some sort of conversion, that a 3.50 ERA in the NL isn't the same as a 3.50 ERA in the AL. But that was because of the DH.

Now it may be more overarching than that. Thirteen transitions, totaling 26 player-seasons out of thousands, surely don't tell the whole story. But when just about all 13 agree so strongly, it's hard to ignore the facts.

Maybe "NL stuff" is too easy, too knee-jerk. Or maybe it has a lot of basis in reality.