Foundations of elation generalized quadrangles

In these notes we are interested in the following fundamental question: "Given a thick generalized quadrangle SW with elation point (respectively center of transitivity) x, when does the set of all elations about x form a group?". It was the general belief for a long time that this is usually the case. However, the goal of these notes is to show that this belief is wrong. To that end, we will answer the question for all known (classes of) generalized quadrangles. (c) 2004 Elsevier Ltd. All rights reserved.