Joinery

While trying to salvage a canvas stretching frame that must be more than 100 years old, I encountered a miter joint I’ve never seen before. What is it called?

Is there a list of tried-and-true doweling rules?

Q:

My dovetail jig manual suggests using a dead-blow mallet on tight joints. What is it, and why should I buy one?

The latest thing on my bench is a chair with a few loose legs and stretchers. One stretcher even pulls right out of its socket. How do I tighten the joints?