Break angles
 

In order to establish a fixed baseline , I choose the Martin D-18 . This has got to be a familiar platform to use .
At what places do string break angles exist ?

Nut and saddle.

One could argue at the wrapping around the tuning pegs, but it’s relevance depends on the topic

In reference to the saddle, there is what is commonly referred as a break angle, which is the angle of the string length from saddle to hole in relation to the "speaking" string length. But to prevent the saddle from prematurely wearing, the top of the saddle where the string rests usually has an angle shallower to some degree than that of the former - the "ramp" so to speak. This could occur at the nut as well.

I'm in agreement with those two. Not that I am a great authority though.

Please elaborate .
Thank you .

I would say at each end of the speaking length of the string, at the deflection over the nut and the deflection over the saddle, measured relative to the at rest position of the speaking length of the string.

What Roger said.

I did some experiments on this several years ago, and found that you don't need nearly as much break angle as some folks seem to think. The point of the break angle is to 'fix' the end of the string and define it's speaking length. So long as the string doesn't hop off the top of the nut or saddle as it vibrates, or roll sideways, it will 'know' how long it is, and what note to make. All the vibrating force will be transmitted to the guitar, and more break angle won't increase the signal. So far as I can tell, something like 12-15 degrees of break angle should be 'enough'; any more than that is superfluous.