Before coins started to be used systematically, the value of transactions was measured up in kind, depending on the region. Thus, in Draco's Athens, in the first written code of laws in 621-620 BC, certain offences were punished by imposing a fine, whose value was measured with the bous (ox). In Solon's time the medimnos (bushel) was a rather stable measure. Each citizen's property -regardless of its source- was estimated on the basis of medimnoi of grain or wine. In the famous inscription from Gortyn in Crete, which dates from the 3rd century BC, there are references to the code of laws which was in force in the 6th century BC. It is inscribed on it that fines and deposits should be measured in lebetes, that is in metal vessels.
Coins replaced an older system of transaction based on iron spits, the obeloi. These obeloi resembled cooking spits and they were used for roasting animals, as well as a medium of transaction. They were very slender, so it was possible for someone to hold six of them together. It is maintained that the word obol -which refers to the lowest denomination of drachma- derives from obelos (spit), while the word drachma derives from dratto (to seize) -drax, meaning a bundle of six spits.
Since the moment metal was used in order to facilitate the exchange of products, it was converted into a kind of currency. Later, when it was used as a specific unit of weight, it acquired the meaning of currency, as this is perceived nowadays. Finally, once metal was stamped with an emblem, it became a currency, that is a coin, and its use was analogous to today's use.