Not sure why this is not shown in any documents, but there is something you need to be aware of that is just as important as not cutting the quick.

There is a nerve that runs through the nail and it is extremely painful if cut. Sometimes the nerve extends further down the nail than the quick and sometimes the quick shows first. I imagine that having had this cut is the reason some dogs hate nail clipping. You should grind nails if at all possible as you can see this nerve emerge as you grind.
The person that posted this probably had no idea what they were showing. Look at the center of the nail and you will see what appears to be a white “pimple.” It is not, it is the nerve, as opposed to a red/black/brown dot(Yeah, I’m colorblind. It is a dark spot) which would be the quick. Why does it stick out like that one might ask. The nerve will not grind. Well, it does if you work at it, but by then the dog should already have started to chew your arm off! It looks a bit like a piece of monofilament fishing line sticking out.
If you are cutting the nail with clippers, you either cut the nerve or you don’t. If you do, the dog will definitely let you know as nerve pain is one of the worst pains any living creature can experience. (except for kidney stones, but I digress)

If you rub that nerve with your finger you will see the dog react as it is extremely sensitive.
The person working on this nail did the right thing. Obviously these nails have been ground, not cut. They also have been ground around the edges so when they came close to the quick or the nerve they would know to stop. This one toe has probably been ground just a little too far as that nerve is fully exposed. This is just one of the reasons I want to position the foot so I can see the bottom of the nail as I grind. Not only can I see what is being ground, but the heat created by grinding gets to that nerve and the dog will pull its’ foot back.

Now for a bit more about doing nails

You can go search about cutting nails and you can get all the basics. I am writing this to tell you what I have learned while grooming for 20+ years and am surprised no one else has written about.

The next item I want to point out has to do with black nails. When you look at a black nail you will see that there are two areas divided by color. One part is flat black and the other is shiny black. What does this color difference mean? The shiny nail is the “live” part of the nail, while the flat portion of the nail is “dead”(yes, technically the whole nail is dead, I am not an idiot) Barring any of the other well written ways to tell where to cut a black nail, you can use this color difference to tell about where to cut. Cut slightly beyond the shiny part leaving a bit of the flat black.
Sometimes it is hard to see the change as it can be subtle, but it is there. I will try and get a good picture of this and update this post with it.