This Sennheiser extension cable (article 508649) is a 6.3 mm (1/4 inch) jack extension for the XS Wireless Digital (XSW-D) instrument system; Sennheiser names it the CI 63 (the supplier lists it as CL 63). The XSW-D instrument transmitter plugs directly into the output jack of a guitar or bass, which does not suit every instrument: some jacks are recessed, angled or sit on the top edge of the body where a plug-in transmitter gets in the way. The CI 63 places a short cable between the instrument and the transmitter - a 6.3 mm plug at one end and a 6.3 mm socket at the other, as Sennheiser's photo shows - so the transmitter can be clipped to a strap or belt instead. It is equally useful on a pedalboard for positioning the XSW-D receiver. Sennheiser lists it as compatible with the XSW-D Instrument Base Set and XSW-D Pedalboard Set. The supplier gives the length as 0.6 m. Useful for guitarists, bassists and worship bands running digital wireless.