Can you cite other master's theses when writing your own? I am completing a master's by research (in Humanities), and thus shall be writing a thesis with no class work. I am using both a mixture of primary and secondary source but one item that has me a bit unsure is the use of other master's theses. Is there an expectation at the master's level that you would only consult documents at a higher level (PhD and up)? I know the simple answer would be to ask your advisor, which I intend to do, but I wondering it their any accepted academic practice or principle on the issue?

Should or can I refer to (and cite) other master's thesis as source material or should I be only using material that would be considered above the master's level, such as PhD theses?
In general, there is no such widespread requirement or expectation anywhere in academia. (Perhaps a particularly unusual advisor might have such an expectation, but even then, I doubt it.)

If you note the citations of research papers, a decent number of them will cite master's theses—some of my papers certainly do! And if I can submit a paper to a major journal that cites a master's thesis, why wouldn't another student's master's thesis?