Commonly, antique French spindles can be found with metal caps. The caps helped to support the spindle if two hands were desired for drafting, or allowed for the spindle to be used suspended.
Copper caps can be ordered to fit a spindle you already own or for a spindle to be made.
This design is called a pigtail cap and caps are removable/tunable. Designed as a spiral, they lean either right or left (clockwise/counterclockwise).
French spindle with spiral cap being used suspended, demonstration.
Copper caps are useful for both supported and suspended spinning. While a spindle is being used supported, the cap ensures that the spun single comes to the very tip of the spindle every time. It reduces drag sometimes caused by the spun single rolling over the spinning tip (making spinning at a variety of angles easier and efficient).
In a test, I spun equal amounts of fiber on the same Russian spindle with and without a cap using a double-drafting method. The output was 20% more with a cap than without.
Caps can also save your spindles from impact damage. The cap acts as a hook. If the spindle ever gets away, it just hangs instead of bouncing out of reach and possibly on to the floor.
Care must be used when utilizing the cap to turn a supported spindle into a primarily suspended one. Caps are removable and can come loose.