Three-Way Switch Drawing

I went looking this morning for an Allen wrench in a canvas bag that I've used for 45 years as my main toolkit. At the bottom of the bag where I found the Allen wrench, I also found this crumpled piece of 1976 paper when I spent a year or so as an electrician's helper. I could never get it straight in my head how two switches could operate to turn on and off the same light. So my father, the electrician, explained it to me and I drew it as a memo to myself. I turned to this diagram many times during my aborted apprenticeship, and a few times since.

I haven't done much wiring in recent years. This morning, I hadn't looked at the diagram in twenty years.