These data furnished a basis for calculating rates of postage, when the limitation of the zones had been determined, a matter of some little difficulty. The first, and most obvious, suggestion was that the zones should be reckoned as from each post office, but in view of the fact that there were some 60,000 post offices in the United States, grave practical difficulties would arise with a scheme providing for special rates from each post office. It was then thought the State might be adopted as the unit of area, but the States differ so widely in area that such a system would have resulted inequitably, giving equal postage charges for very unequal services. A citizen in Texas or New York State, for example, would be able to send his parcel many times as far as a citizen of Massachusetts or Delaware. The same objection applied, though not in so great degree, to the county as a unit of area. Moreover, these units would be liable to arbitrary change. Failing the discovery of any satisfactory unit based on the political divisions, recourse was had to purely theoretical divisions, based on the imaginary lines of latitude and longitude. The actual degree parallels were rejected as the limits of the units of area because, within the quadrangle formed by them, there would be a maximum zone distance of some 80 miles; while at the same time, for transmission between two places perhaps only two miles apart but lying on different sides of the line, postage would be chargeable as for the second zone. This difficulty was overcome by an ingenious suggestion, which emanated from the department, to divide the degree quadrangles in four, that is, into squares of 30 minutes dimension, and to allow the sending of parcels at the first zone rate from or to places in every contiguous quadrangle. Under this method, which was adopted, the United States is divided into 3,500 units of area, which are definite, never-changing, and practically uniform in size, the slight difference in area depending on the distance from the Equator being negligible. Each unit is given an index number, and all post offices in the unit have the same index number. Each unit has its own zone limits, which apply equally to every office in the unit. So that in order to discover for what zone postage is to be charged between two places, it is only necessary to ascertain what are the zone limits for the units of area in which the places are respectively situated. A simple reference to a guide showing in what unit of area the post office of destination falls, and a consultation of the zone map of the office at which the packet is posted, that is, a map showing the boundaries of each zone measured from the unit of area in which the office is situated as centre, gives in a very short space of time the rate applicable to the parcel.