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I participated in a legal society at my school for all three years. I was the treasurer during my second year and was elected as a co vice-president before this year started. But I resigned from the latter position earlier this semester due to personal emergencies that had happened one after another since the beginning of this academic year.

Am I allowed to mention the co vice-president experience when I list the legal group on my resume (in just one line), or should I stick to describing myself as the treasurer instead?

You were co-VP for a semester, right? You could list something like "vice-president - Sept 2022-Jan 2023" [or whatever month], with or without an explanation (like "(resigned early for personal reasons)" or some more elegant way to word that). I don't think it's going to make a lick of difference to your outcomes, but you're certainly entitled to.

The main issue is whether it's going to raise questions about why you resigned early and get into stuff that you may not want to get into in a job interview. If you don't want to get into any of that, then don't list it.

If I'm misunderstanding and you never actually served, there are probably ways to word that on your resume but I would see no point at all in doing so.