Actually, even back in 1981, there were multiple lines, though not nearly as much as now. Here's a list of the sets released in 1981: http://brickset.com/sets/

Actually, even back in 1981, there were multiple lines, though not nearly as much as now. Here's a list of the sets released in 1981: http://brickset.com/sets/year-1981 http://brickset.com/sets/year-1981 . Out of 58 sets, 16 are designed for generic use. Much of the rest are buildings and vehicles from the modern world, like houses, fire stations, tow trucks, ferries, helicopters, dune buggies, etc. But there are also sets with medieval knights and space themes. Plus, Lego created their own characters for a theme they called "Fabuland". There was even a tie-in series: https://www.youtube.com/watch?v=keJua4D2Ymg https://www.youtube.com/watch?v=keJua4D2Ymg In summary, back in 1981, there were generic sets plus four different themes.