Page 424

424 A HISTORY OF THE CRUSADES 
 In the Moslem world and almost all pre-modern economies, current gold and
silver coins were related in value only by the marketplace, like different
national currencies today. Governments could fix values only in certain transactions
(for example, in denominating salaries in one currency while paying them
in another) and sometimes might attempt to decree relationships in private
transactions, but such decrees were widely evaded and impossible to enforce.
In normal times values were fairly stable, so that people could have a notional
rate for the relationship of the dinar and the dirham, but in any substantial
transactions involving both coins the exchange rate had to be set by negotiation.
The value of foreign coins in local currency was also determined by supply
and demand, not by precious metal content; moreover, if payments were made
by weight, local weights would have been used to measure foreign coins, so
that varying coin weight standards would have no effect on value. To be sure,
metal content set a floor value for any coin, but its actual equivalency
in local currency was set in the marketplace. 
ISLAMIC GOLD COINAGE 
 The major Moslem mints for gold coinage in the vicinity of the crusaders
were Cairo and Alexandria in Egypt, and Mosul and Baghdad (named Madinat
as-SalAm on coins) in Mesopotamia. A few other mints in the region issued
gold occasionally. For gold coinage, the Euphrates was a clear dividing line
between two different systems. The dinars of Mosul (p1. XII, no. 1) and Baghdad
(p1. XII, no. 2) did not circulate in Syria and did not influence the coinage
of the crusaders. The issues of both these mints continued the Selchükid
Iranian tradition. For the eleventh and first half of the twelfth centuries,
their coins are rather scarce, but after about 1160 the representation of
both becomes more continuous, while for the thirteenth century (until the
Mongol conquest) the coins of these mints are common, large, well engraved,
and quite pure, although completely irregular in weight,5 a general characteristic
of gold coins east of the Euphrates from the tenth to the fourteenth centuries.
 Egypt first minted gold coins under Islam in 786 and immediately became
one of the principal centers of gold coinage in the Moslem world, issuing
dinars continuously and abundantly throughout the ' Ab 
5. Arlette Nègre, "Le Monnayage d'or des sept derniers califes abbasides,"
Studia islamica, 
XLVII (1978), 165—175.