The app lists hundreds of knots, from the Albright knot to the Zeppelin bend, along with easily followed animations. These can be stopped and started at any point so you can examine them. If you see a knot that is the reverse image of how you tie it, tap the little icon at lower left and it flips the knot. There’s an Info tab for each knot that explains its uses, origin (if known), variations, similar alternatives, and whether, when, or why the alternatives should be used. I’ve spent hundreds of hours playing with this app the way others play Candy Crush or whatever’s most popular this week. I’ve spent hundreds of hours on it, often creeping out whoever’s next to me on the airplane.