BTW, others have answered your question, and usually stackable switches, of the same model series can be stacked, although the requirements of degree of matching hardware, IOS version, its release, its feature set and, possibly, other criteria, can vary. In other words, you really need to always check what's allowed, or not.
As a historical example of how varied/flexible stacking switches can be, the original 3750 series eventually had two different stack cable bandwidths, didn't require matching feature sets, didn't always require matching IOS versions, and had one specific model (3750G-12S) that supported both a common and specific model SDM templates, the latter only usable in a stack of just those specific models.
