Morphology Q: 'Under' - free or bound?

Hi everybody,

I'm currently studying morphology as part of a linguistics degree, and I've came across the word 'underscores'. I'm trying to figure out all of the morphemes, and whether they are free and bound. Obviously the 's' is bound, and the 'score' is free, but I'm not sure whether 'under' is free or bound?
My reasons for questioning this is because 'under' can stand alone as a meaningful unit of language, and therefore I would assume that it is free; however I do not believe it is a compound word, where two free morphemes merge together e.g. blackboard, as it just doesn't seem right? If this make sense, 'under' almost seems less important than 'score' but I just can't work out whether it is free or bound.

If anybody has got any additional information, or if I've confused some of the information that I have discussed, please let me know - I'd really appreciate it.