Introduction -- The present volume is the fourth in a series whose object is to teach chess players to think for themselves. This is not so easy a task as it may seem; in fact, many will wonder why the task is necessary. There is a definite need for such books, I feel, because most players fall into two groups: those who like to study books, but, getting little opportunity to apply their theoretical knowledge, generally prove sad failures in actual play; the other type of player who disdains or ignores book knowledge, relies on his own ideas, and consequently lacks that basic familiarity with broad general principles without which considerable chess ability is impossible.