Legal Question in Intellectual Property in California

You'll find numerous examples of 3rd party "plug compatible" or even downloadable software products that seem to fit the description. Also, I can plug my Thorens turntable into my McIntosh amplifier. There was a time when the old Bell System tried to keep people from adding accessories to their telephones....I once had an answering machine that avoided Bell's wiring by mechanically lifting the receiver and acoustically listening to and recording the incoming message. I think this was based on a threat of contract invalidation rather than any law, but in any case we now have all kinds of gear that attaches electrically to the phone line. So, I can't categorically state that there are NO laws - there's probably one or more somewhere on the books, protecting this or that special interest. However, if you look at the huge number of user-connectable devices from different manufacturers, I think it's readily apparent that most accessory products are legally OK.