Abstract: In the preferred and illustrated embodiment of a capo, such a capo is set forth as comprising a capo for a guitar, banjo or similar stringed instrument including an arm for engaging the instruments' strings; said arm having a contact piece which distributes the pressure on said strings by means of hydraulic pressure, and including a body, for clamping said arm to said strings, which is substantially made of spring like material manufactured in such a shape so as to cause the rear surface of the neck piece of said instrument to function in the manner of a camshaft when the capo is installed, and including a locking mechanism which prevents the accidental disengagement of the capo. The capo is designed so as to allow the installation of the capo to be achieved with one hand, and without requiring the shifting of the instrument from the normal playing position.