Wachumi wanagawanya aina za sarafu za madini katika aina mbili kuu: kiwango cha chuma kimoja (single metallic standard) na kiwango cha chuma mbili (dual metallic standard). Aina ya kwanza ni pale sarafu kuu zinapowekewa mipaka kuwa sarafu ya chuma kimoja tu. Kwa upande wa pili, yaani kiwango cha mbili, sarafu zote za dhahabu na za fedha ndizo zinazowakilisha sarafu kuu. Kiwango cha chuma mbili kinahitaji kuwepo kwa sifa tatu.

Economists divide the types of metallic currencies in to two main types: the single metallic standard and the dual metallic standard. The first is where the main currencies are restricted to one single metallic coinage. As for the latter i.e. the dual standard, both the gold and silver coins represent the main currency. The dual metallic standard requires the existence of three qualities:

1. Sarafu za dhahabu lazima ziwe na thamani ya kisheria isiyo na kikomo (yaani, hazina nguvu ya ununuzi iliyowekewa mipaka).

1. Gold coins must have an unrestricted legal tender (no fixed purchasing power).

2. Hakipaswi kuwepo na vizuizi vyovyote katika kutengeneza sarafu kutoka kwenye madini ya bullion ya metali zote mbili.