T 0 paraphrase a former President, it is altogether “fitting and proper” that a dentist who has taken great pains to make satisfactory trial dentures should be greatly concerned that the occlusal relations which he has painstakingly achieved will be reproduced in the processed denture. According to the writer’s experience and also to a recent article in this JOURNAL, his chances of achieving this result are very slim. In almost all cases, there will be some vertical dimensional change in the relationship,of the teeth to the cast. If the space between the teeth and the palatal surface of the denture is increased, this change will be referred to in this article as vertical dimensional increase. If the space is decreased, the term used to describe the change will be vertical dimensional loss. Of these two, the more common is an increase in vertical dimension. It is by no means impossible by present procedure, or by procedure other than that to be described here, to avoid vertical dimensional change. The most commonly practiced method of processing a denture is the use of methyl methacrylate, monomer, and polymer in a compression molding technique. To date, this technique yields the optimum properties of methyl methacrylate. The author believes that by very careful and repeated trial packing the increased vertical dimension that usually results can be greatly reduced, if not eliminated. However, the time and care that such repeated trial packing would necessitate are beyond what may be expected, even of competent technicians. It is the author’s purpose to discuss briefly the causes of the shift of the teeth from their original positions and to disclose a remedy. Contrary to his usual experience, the writer does not find the difficulties to arise from remote or unexpected causes. Instead his obs&at&ns indicate that incomplete and often uneven closure of the Bask are important factors in displacement of the teeth during processing. Often the flask is completely closed, but with such force that cast distortion and investment dikbrtion occur. It also appears that sometimes repeated opening of the flask and repeated packing may loosen the teeth from the investment. If these factors are, indeed, the main causes of the increase in vertical dimension that ‘takes place during processing, the steps that would eliminate or reduce the following items, incomplete or inaccurate closure, excess pressure on the ca$, and trial packing, should have like effect on the errors attributed to these causes. The contrary may be accepted with very little reservation. Other causes only lightly dealt with here are the almost primitive and inadequate flasks and presses now in use.