Fuji Rod Caps fit a wide range of tenkara, keiryu and seiryu rods. Use the PTC-10 for the thinnest rods, KTC-12 for rods that are a bit thicker, the KTC-16 for thicker rods and the KTC-20 for most 6+m keiryu rods. The number tells the diameter in millimeters. Measure the diameter at the end of the grip section to decide which rod cap to get.
The Fuji Rod Caps are more secure than tip plugs and harder to lose. They protect the rod tip when the rod is collapsed with the line still attached.