Book I is titled "Problems Which Can Be Constructed by Means of Circles and Straight Lines Only." In this book he introduces algebraic notation that is still in use today. The letters at the end of the alphabet, viz., , , , etc. are to denote unknown variables, while those at the start of the alphabet, , , , etc. denote constants. He introduces modern exponential notation for powers (except for squares, where he kept the older tradition of writing repeated letters, such as, ). He also breaks with the Greek tradition of associating powers with geometric referents, with an area, with a volume and so on, and treats them all as possible lengths of line segments. These notational devices permit him to describe an association of numbers to lengths of line segments that could be constructed with straightedge and compass. The bulk of the remainder of this book is occupied by Descartes's solution to "the locus problems of Pappus." According to Pappus, given three or four lines in a plane, the problem is to find the locus of a point that moves so that the product of the distances from two of the fixed lines (along specified directions) is proportional to the square of the distance to the third line (in the three line case) or proportional to the product of the distances to the other two lines (in the four line case). In solving these problems and their generalizations, Descartes takes two line segments as unknown and designates them and . Known line segments are designated , , , etc. The germinal idea of a Cartesian coordinate system can be traced back to this work.