These rates remained in force until 1759, when a variety of causes led to a further increase of rates. The Seven Years' War had made an increase of taxation necessary; there had been a depreciation of money, and an increase in the cost of all commodities, which had resulted in an increase of the expenses of conducting the posts. Under the tariff of 1759 the eight zones of 1703 were maintained, and the rate for single letters varied from 4 to 14 sous, with an additional rate of 1 sou for all letters enclosed in an envelope. The principle of charge according to weight was introduced for letters weighing less than 1 ounce, which up to this time had been charged only according to the number of sheets. Double letters weighing more than 1/4 ounce and less than 1/2 ounce, were rated at 7 sous for the first zone, and for the other zones a "rate 2 sous less than the ordinary rate for double letters." Packets were charged by the ounce, and the rate per ounce was four times that for a single letter. As with the tariff of 1703, distances were calculated according to the route actually followed by the couriers. No further modification of the rates was made until after the Revolution. The lease of the posts was due to expire on the 31st December 1791, and it was decided that the Legislature should rectify the tariff before the date at which the posts would revert to the State.[189] A rectification was accordingly announced by the decrees of the 17th-22nd August 1791. This revision slightly increased the rates of 1759. The initial rate of 4 sous for single letters circulating within the same _département_ was retained; the rate for letters circulating in the same _arrondissement_ was fixed at 3 sous; between _départements_ the rate was increased, and varied from 5 to 15 sous, according to distance of transmission; and the number of zones was increased to eleven.[190] Distances were no longer to be reckoned according to the length of the route actually traversed, but from point to point as the crow flies. The points were not, however, the actual points of posting and of delivery. In each department a point was fixed upon, and the rate for all letters posted or delivered in the _département_ was calculated as from that point; so that for a given weight the same rate was payable on all letters exchanged between the same two _départements_. This system, though comprising a very large number of rates, was much simpler than the earlier systems. Any one town or village now had only 82 rates for each step in the scale of rates, whereas under the previous system a special rate must be calculated for every other town or village in France. To assist the application of this tariff, a map showing the central point fixed upon for each of the 82 _départements_, and the distances from each central point to all others, was prepared and supplied to every post office in France.