Thumb indexing can be done more or less thoroughly (expensively). This picture shows the cheapest way, just printing a locator near the outer margin, varying vertical alignment from section to section so that when the reader riffles through the book, some sort of indication of where they are will show up. This basically costs nothing. Placing the type in the margin of every page and using a tiny bit of ink to print them really doesn’t cost anything noticeable. I have also seen this done with the boxes extending further out so that they actually trim off in binding. This means that a shadowy gray smudge shows up even when the book is closed. And this can be quite effective. It’s all a matter of the budget for the book. Publishers’ manufacturing departments are used to having to shave money off the budget at the last minute. By the time the book reaches us people up-stream have merrily overrun their own budgets and so it comes down to the manufacturing backstop to salvage the cost picture. Thumb indexing, would be an obvious place to look for savings.
Real thumb indexing involves using a hydraulically-powered clipper gun to cut a semi-circular bite out of the edge of a bunch of pages. I find it hard to believe that this could ever be automatable: the pages have to be riffled before the chopper does its work, so that a bigger bite is taken out of the last page and progressively smaller bites as you get further away. Even before that, the first step in the process which is done on bound books only, is to go through the book and insert bits of paper to indicate where the cut should start and the tab be pasted. The operator then opens the book at each marked location and makes the cut. The fewer tabs you have the cheaper indexing will be, and people often double up their tabs. I have a dictionary I worked on which manages to compress the alphabet into 11 tabs.
This second picture shows the full treatment: a triple rank of index cuts. Note that Zephaniah’s tab appears on the first page of the Book of Zephaniah. Sometimes the publisher will seek to save money by allowing the tab to be placed near where it ought to fall: if the Acts tab falls anywhere near the beginning of Acts, this would be acceptable. However if you are going to do thumb indexing it seems to me you might as well go all in: saving a few pennies just looks cheap and nasty.
As you can see in the third picture, even a de luxe thumb indexing job on an expensive Bible will combine some of the tabs. Many books of the Bible are just too short to warrant their own cut.
I believe this book was indexed at Ross-Gage in Indianapolis. Their website almost unbelievably offers a video where you can see the world’s first automated thumb cut machine. They also illustrate other indexing methods, some of which I wasn’t aware of. I don’t think I’ve ever seen a corner step edged indexed book.