Bei ihnen zu Hause ist es anscheinend noch so, wie ich es noch vom Haus meiner Großmutter kannte. Wen zu sein soll musste man den Schlüssel umdrehen. Ansonsten konnte man mit einer leichten Drehung des Knaufs den Schnapper zurückziehen.
OK. I'll try to explain the US lock system again. (It's been done in a fair number of threads, not always with great success.)
The home I grew up in, built in 1950 in a quiet suburb, simply had doorknobs with a built-in lock in the doors. The lock could be opened from the outside with a key at any time. There was a small knob in the center part of the inner doorknob. This small knob could be turned so that a) the door would be locked from the outside automatically whenever it was shut, or b) the door would remain unlocked unless you locked it with a key from the outside. In either position, you could always get out of the house without a key. We normally kept it in position b except at nights, when we would switch it to a. When we left the house and wanted to be sure that the door was locked, we used a key from the outside. This was a very simple type of lock without any sort of deadbolt function—but break-ins were unknown when I was growing up there.
If you had that type of lock but wanted additional security, you could add an additional deadbolt lock to the door. These usually required an additional key because they were retrofitted. There were two types of deadbolt locks: Those that required a key to be opened (and locked) from the outside but had a knob that you simply turned on the inside to lock/unlock them, and those that required a key both inside and outside. (I added the latter to the doors of a 1912-era home that I bought. The doors had glass panes, and the absence of a knob on the inside meant that it wouldn't be as easy to open the door simply by breaking one of the panes.)
Nowadays, lock sets often come with an integrated deadbolt so that the simple lock and the deadbolt are opened with a single key. However, the option still remains of having the door lock automatically when you pull it shut or having it remain shut but unlocked unless you lock it with a key.