we just don't, there is no fixed top and bottom roles in relationships between two cis women in most of the cases but it happens sometimes, there is only submissive and dominant roles in yuri, the dominant one is called tachi and the submissive one is called neko 1 reply
"On Wikipedia disambiguation page for "Seme" one of the suggested usages of the term is:
Seme, a manga/anime term for a dominant partner in a homosexual relationship
The first paragraph of that "seme" link says:
The two participants in a yaoi relationship (and to a lesser extent in yuri) are often referred to as seme ("top") and uke ("bottom").
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