According to what I've read, in earlier homes (constructed in the 1800s), the family utilized two doors: one for everyday family functions and the other to enter the more formal portions of the home (such the parlor) when visitors arrived. As time went on, this practice was discontinued due to security issues. However, it wasn't until the 1920s that this method was actually abandoned for one door. The reason for this is because cars were not available to the general public in any significant number until the early 20th century, so people were not concerned with unauthorized persons entering their homes. Here's how the process worked: When a visitor came to the house, the butler or maid would notify the master of the arrival. The family member who had not been visited upon first arriving at the house would then proceed to open either a front door or back door as appropriate. Once everyone had been greeted, both doors would be closed again and the family member(s) who had opened them would remain inside while the rest of the family went about their business. At the end of the day, whoever was still awake would once again visit each door without opening it to ensure that no one had come into the house while they were asleep. The butler or maid would know which door to use based on the purpose of the visit. If it was just "to see how the family was doing," then both doors would be opened.