(Schuman, Verne B) The Seven-Obol Drachma of Roman Egypt (Classical Philology, Vol. 47, No. 4, Oct 1952, pp. 214-218)
http://www.jstor.org/stable/265590
Interesting ...[214] It has long been accepted as an established fact that no coin with a value of seven obols existed in Roman Egypt, a view which seems to be supported by the numismatic evidence. For, apart from a few didrachms and drachmas issued by Claudius,1 the only silver coins (billon) current were the Alexandrian tetradrachms; yet a 7-obol drachma, at times termed “silver,” is found in various documents.2 This apparent inconsistency between the numismatic and documentary evidence makes it appear that the value of the tetradrachm fluctuated between twenty-eight and twenty-nine obols; hence the 7-obol drachma was merely an accounting device used to denote one-fourth of a tetradrachm whenever its value was twenty-eight obols. This conclusion, however, is found to be lacking somewhat in logic. In a fiduciary system, such as existed in Roman Egypt, there is no good reason why the relationship between one denomination and another should ever vary. Consequently, the ratio between the tetradrachm and obol should remain fixed. But aside from all logic the evidence of P Lond., 131, a farm account of A.D. 78/9, must be considered.
In this document the account of receipts was kept in drachmas of silver, the account of expenditures, for the most part, in drachmas of copper at six obols to the drachma. To get receipts and expenditures on the same basis at the end of the month the copper drachmas were converted to silver.
...
To coordinate this interpretation with the statement quoted above in which copper is converted to silver at two different rates, consideration must be given to the probable procedure involved in handling money on two different bases, silver and copper, the former at 28 or 29 obols to the tetradrachm.
It is a reasonable supposition that [215] money received in silver at two different rates was kept in separate containers. Otherwise there would be no logic in denoting some receipts as silver, some not, when all were added and totaled as silver. Almost daily expenditures for labor must have necessitated a constant changing of the silver received into small coin (rated at six obols to the drachma) for the payment of individual wages which were a few obols a day. If a tetradrachm was converted into change it could only be at the rate of twenty-nine obols. But if 7-obol drachmas were to be converted, purely as a matter of consistency in procedure, this might be done in units of four. The fact that in this one document they were converted in units of four in no way implies that it could be done only in that way. Furthermore, since use is made of a 7-obol drachma in this same account as indicated earlier, proof weighs far more heavily in favor of a 7-obol drachma than a 28-obol tetradrachm.
...
[216] Not only does the existence of a 7-obol drachma allow the accounting practices in the preceding two documents to be explained in a logical way but it also provides a reasonable explanation of the αι καί formula found frequently in tax receipts and registers. Since I have discussed this elsewhere9 one example will suffice here.
...
To summarize, if the evidence presented here has been correctly interpreted, we may conclude that there was no 6-obol drachma coin on the silver standard; that there was a 7-obol drachma termed “silver” though actually made of copper; that the other copper coins minted after Augustus had values of 4 obols, 2 obols, 1 obol, and 1/2 obol; that Augustus issued copper coins valued on the silver standard at 2 obols, 1 obol, 1/2 obol, and 1 chalcus; and finally, that there existed under Augustus a 29-obol tetradrachm and a 7-obol drachma although these coins were not issued by him but were part of the existing Ptolemaic coinage.
DCH