Trigger modi of an oscilloscope

I'm sure this belongs to the "every once in a while asked again" questions, but I've read several posts on this site and also on other sites and haven't found an answer to two of my questions, yet.

LINE-Trigger

The LINE-Trigger is supposed to trigger whenever the signal has 50 Hz as frequency. It makes sense to me that 100 Hz, 200 Hz, and so on are triggered, since instead of one period it does 2. However, when I put in an 75 Hz signal into the oscilloscope it still triggered and I dont' understand why. The points where the 50 Hz signal is 0 are also points where the 75 Hz signal is 0, but while the 50 Hz one is increasing (if it's a sine-signal) the 75 Hz isn't. So does it really only matter that the signal is 0 at time 0 and then after 1/50 seconds it's zero again?

EXTERNAL-Trigger

The second question I have is about external triggering. According to the sources I read "the external signal is used to trigger". For me, this sounds as if the oscilloscope triggers, when the input signal is exactly the same as the external trigger signal. However, I used a function generator and used the external trigger output and also let the oscilloscope display the signal, it was a square voltage. I then gave a sine input (which I wanted to use the external trigger to trigger for).

(For me) surprisingly, it still triggerd even tho the sine signal had a different form than the external trigger (square voltage). The sine signal was increasing when the square voltage was positive and the sine signal was decresing when the square voltage was negative. So not only the shape was different but also when the sine signal reached it's peak, the external trigger jumped from negative to positive, so the 0 points didn't match. The only way I could explain this to myself is that the oscilloscope only triggers when the frequency of external trigger and input signal are the same.

Sorry for that much text, I just wanted to show you that I have already thought about this and I'm not just to lazy to read other posts.

Answer

If you use the line trigger or the external trigger then the frequency and form of the displayed input channel does not matter. The scope doesn't trigger when the line or external signal are the same as the displayed signal, it triggers when the chosen trigger signal passes through your selected trigger level with your selected slope. If you are looking at a signal that has a different frequency from the line or external signal then the scope will seem to be triggering at random. You should only use these modes when the signal you want to observe is derived from or strongly related to the line or some external signal.