Ladies frame (AKA step through) has a second tube that is typically parallel to the down tube. It goes from the head tube to the seat tube and stops.

A mixte has dual stays that start at the head tube, and go straight towards the rear axle. They pass the seat tube and continue to the rear dropout. The rear triangle of a mixte as a result had 3 sets of stays instead of just 2. There is an alternate version with the dual tubes between the head tube and the seat tube are replaced by a single tube.

Many manufacturers if not most made mixte frames in the 70s and 80s,