287, 428.] The diverse goods bartered for slaves were rated by units of value which varied in the several trade centers.

On the Gold Coast it was a certain length of cowrie shells on a string; at Loango it was a "piece" which had the value of a common gun or of twenty pounds of iron; at Kakongo it was twelve- or fifteen-yard lengths of cotton cloth called "goods";[10] while on the Gambia it was a bar of iron, apparently about forty pounds in weight.

But in the Gambia trade as Moore described it the unit or "bar" in rum, cloth and most other things became depreciated until in some commodities it was not above a shilling's value in English money.

Iron itself, on the other hand, and crystal beads, brass pans and spreadeagle dollars appreciated in comparison.

These accordingly became distinguished as the "heads of goods," and the inclusion of three or four units of them was required in the forty or fifty bars of miscellaneous goods making up the price of a prime slave.[11] In previous years grown slaves alone had brought standard prices; but in Moore's time a specially strong demand for boys and girls in the markets of Cadiz and Lisbon had raised the prices of these almost to a parity.