When a variant was noticed, it might be introduced into the new copy by correction, or it might be noted in the margin or between the lines, preceded by some such expressions as ἐν ἄλλῳ (κεῖται), ἐν ἄλλοις, ἤ, γρ. (= γράφεται), al. (=alibi or aliter), vel. When a copy furnished with this kind of primitive scribal apparatus served in turn as an exemplar to another scribe, he might do any of four things. He might preserve both the variant in the text (t) and the marginal variant (v) in their places; he might retain t and omit v; he might adopt v in place of t, without mention of t; or he might put v in the text and t in the margin. This confluence of readings from more than one exemplar is known as contamination.
(West 1973, 13)