Schaimpflug's rule.

This is the rule on tilting the easel and carries on from my last post.  The most accurate way of correcting distortion is to use Scheimpflug's principle: This states that you should tilt the negative as well as the baseboard and that they need to be canted over in opposite directions to each other. Additionally the incline should be such that the plane of the negative and the plane of the baseboard should meet in the plane of the aperture to give maximum sharpness over the whole image. 

You will find that the more expensive enlargers allow the plane of the negative to be tilted, add a tilting easel to the mix and the results produced can be remarkable.

Related posts:

Tilting the easel.