From Wikipedia ...
The higher frequencies may be a bit off, but cassettes were just good enough sounding as things go. I found I could interchange types I & II. You could not do that with type IV.
Rich
Notches on top of the cassette shell indicate the type of tape within. Type I cassettes only have write-protect notches, Type II have an additional pair next to the write protection ones, and Type IV (metal) have a third set in the middle of the cassette shell. These allow cassette decks to automatically detect the tape type and select the proper bias and equalization.
The higher frequencies may be a bit off, but cassettes were just good enough sounding as things go. I found I could interchange types I & II. You could not do that with type IV.
Rich