In a book by German historian Klaus Bringmann, I read that during the regal period, the whole population was divided into three tribes. He furthermore states that each tribe was divided into 10 curiae and plebs. So my first interpretation was that a Roman citizen was either part of a curia or he was part of the plebs. But in either way he belonged to one tribus.
However, on the English Wikipedia article about "Curia" it readssays that "later every Roman citizen was presumed to belong to one". Was that something that changed during the republican period or was every citizen part of a curia from the beginning?