Early in the development of every oil field, the operators of relativelylarge tracts of land must decide upon a spacing plan for the wells to bedrilled upon their respective tracts. Usually it is desirable to have a systemof regular spacing laid out over the entire tract, making such adjustments asmay be necessary near land boundaries. Unless the dip of the producing formation is very steep, the equilateraltriangular arrangement is the one most generally preferred, for the very goodreason that it gives the maximum amount of drainage area for the wells up tothe limit at which mutual drainage interference begins. Assuming that the equilateral triangular pattern is to be used, there stillremains to be decided the matter of distance between wells. Many variablefactors enter into the problem of proper spacing, some of which are physicaland some economic. It is not the purpose of this paper to discuss thesefactors, as that has been done in several comprehensive articles during thepast few years. In the end the decision must be based upon an estimate of therelative importance of various factors which apply to the field in question andto economic conditions in the oil industry at the time the development isgetting under way. Even if the best possible spacing distance should be chosenwhen the problem arises, changing economic conditions and variation in physicalconditions in different parts of the field may cause the original choice tobecome more or less unsatisfactory. Obviously, it is advantageous, once a spacing plan has been adopted, to avoidalteration of it as the field becomes better known or as conditions in theindustry change, and yet be able to vary somewhat the acreage per well inextending the area of development. Or, it may be desired to provide for anultimate spacing considerably closer than conditions seem to warrant early inthe development of the field. With these contingencies in mind, it is ofinterest to examine the possibilities which the equilateral triangular systemaffords. Orientation Of Coordinate System In this system the well locations may be considered as the inersectionst ofthree sets of coordinate lines, the lines of each set intersecting those of theother two sets at 60?.