Diocletian's reform of the coinage included the following elements :—
(1) The raising of the weight of gold coins from a norm of about 70–72 to the Roman pound to one of 60 to the Roman pound.
(2) The abandonment of the silver-washed coin now known as the antoninianus and its replacement by a silver coin of the weight of Nero's reduced denarius, i.e. at a rate of 96 to the Roman pound.
(3) The introduction of the so-called follis—a copper coin of about 10 gm. in weight and struck at all ‘reformed’ mints with something very near uniformity of types. The follis was accompanied by fractional denominations.
The date of (1) is discernible (even if it cannot be fixed with absolute precision) by weighings of dated gold coins from a.d. 284, when Diocletian succeeded to power. ‘There was a change in basic standards not later than a.d. 286. One hundred and twenty-seven coins from the mints at Rome, Cyzicus, Lyons, and Antioch dated in the years 284 to 286 show an average weight of 71 grains, indicating a basis of 70 or 72 to the pound. Twenty seven coins from the mint at Cyzicus dated in a.d. 286, twenty-three from the mint at Rome dated a.d. 286–7, and at least ten each from the mints at Siscia and Antioch dated in a.d. 286–9 all show average weights of 82 or 83 grains, indicating a basis of 60 to the pound. No later group containing five or more coins varies more than 5 per cent from this average, except one dated a.d. 296–9 from the mint at Treves.’ The same phenomenon had previously been observed by K. Pink, who associated the change of standard with Maximian Herculius' elevation to the rank of Augustus on 1st April, a.d. 286.
1 I had collected the material for this paper, and had begun to arrange it in its present form, when the brief report of Dr. H. A. Cahn's remarks to the Société française de numismatique was published in the Bulletin de la Société française de numismatique, Nov., 1954, 307 f. As Dr. Cahn's reported views appeared to coincide so closely with those which I had reached independently it seemed worth while to work the problem out in closer detail.
Abbreviations used below are as follows :—
Cahn XIII = Monnaies et Médailles S.A. Bâle, Vente aux enchères XIII (17–19 June 1954). Lallemand = Jacqueline Lallemand, ‘Le monnayage de Domitius Domitianus,’ in Revue belge de numismatique 1951, 89 ff. NNM = American Numismatic Society's Numismatic Notes and Monographs. NZ = Numismatische Zeitschrift. RN = Revue Numismatique. RIC = Mattingly, Sydenham, and others, Roman Imperial Coinage.