Dark green, broad-shouldered, pudgy poblano chilies have an identity problem: They are often labeled pasillas. But chili experts Manuel Rosas of Seminis Vegetable Seeds and Jean Andrews, the trademarked Pepper Lady, tell me that poblanos and pasillas belong to entirely different groups. Pasillas are part of the slender Anaheim group. Furthermore, true pasillas are dried chilacas, a variety rarely seen fresh north of the Mexican border.
According to Andrews, whose latest book is The Pepper Trail: History & Recipes from Around the World (University of North Texas Press, Denton, TX, 1999; $50; 800/826-8911), shape is the distinction between poblanos and Anaheims. Poblano chilies, as a group, are only about twice as long (4 to 6 in. on average) as they are wide, while the members of the Anaheim group (including California and New Mexico chilies) are long and skinny.
For most cooking, you can interchange fresh poblanos and Anaheims ― except for stuffing. Here, only plump poblanos can really pack it in. As for dried pasillas, they can stand in for dried poblanos like mulatos and anchos in sauces, completing the circle.