How tight should the lid be when braising? Much of the braising guidance I've read emphasizes the importance of a tight-fitting lid.  For example, in All About Braising, pp. 18-19, Molly Stevens provides instructions for sealing a braising pot with parchment paper.  The goal is to reinforce the seal since "much of the success of braising depends on trapping moisture in the pot."

On the other hand, Harold McGee, in his "Guidelines for Succulent Braises and Stews," advises braising with "the pot lid ajar to allow some evaporation" (On Food and Cooking, p. 163).  J. Kenji López-Alt, in his Food Lab article on Great Chile Verde Without Hatch Chiles, notes that a pot with its lid left slightly ajar "stays a good 20°F lower, keeping the meat inside at a temperature far closer to the ideal."

How do I make sense of this seemingly contradictory advice?
The advice isn't necessarily contradictory, both methods are valid depending on what you want to achieve. Running out of moisture in a braise is a bad thing, so if I'm braising something with limited moisture then a tight lid keeps the liquid from evaporating. A good example would be when I cook beef shin, I add a cup of water and a cup of wine, and that's all, so I don't want to lose any of it.

In some recipes you start off with more moisture than you need, or there are ingredients which are going add to the moisture in the pot as they cook (think peppers, zucchini, cauliflower). In these cases keeping the lid off a bit allows the excess water to evaporate, thickening up the cooking liquid.