If your doorknobs feel a little loose and jiggly, you may need to tighten either the lock set (for most modern doorknobs) or the knob itself (on older mortise-style passage locks, pictured below).
If you have a modern cylindrical or tubular lock set with a loose knob, see How to Troubleshoot Lock Problems. These knobs either snap onto a cylinder or are integrated into the lock set, so you need to tighten the lock set itself.
If you have a mortise-style lock and lock set (see illustration), follow these steps. These lock sets (both interior and entry models), once the standard, are now rarely used. The knobs thread onto the ends of a square spindle with rounded, threaded corners. A setscrew, located in the knob's sleeve, tightens down on the square face of the spindle, preventing the knob from turning on the spindle when operated.
Loosen the setscrew in the sleeve of one knob with a standard screwdriver or in some cases an Allen (hex) wrench.
Unscrew the knob to expose the spindle and access the setscrew on the other knob. Make sure the setscrew is very tight.
Turn the knob until one of the spindle's flat sides is directly on top. Press the knob toward the door and hold it tightly against the trim (the rose) as you thread the other knob onto the spindle.
Make sure the knob is contacting the trim, but is not tight; then turn the knob slightly one way or the other as needed to align the setscrew with any flat face of the spindle. Then screw in the setscrew until it's very tight.
Test the knob's operation. It should turn freely but not loosely. To adjust the position of the knob, repeat the above procedure as needed, turning the knob onto the spindle more or less.