They had cassette tapes in different materials... Normal, Cr02 (chromium oxide for hi-fi, and metal (for higher-fi)...
Then they were made in 30 min, 60 min, 90 min, and 120 minute tapes... Each side held one half of the rated time...
The trouble with making a mix tape with them is sometimes you ran out of room on the last song for that side and it cut the song off short...
Some higher end cassette deck players also could reverse without removing the tape...