Even though 2.1mm is about 80% of the market, once in a while you’ll find you have and adapter with the LARGER 2.5mm plug and you want to insert it into a jack that has the 2.1mm pin. It’ll fit, but won’t make electrical contact because the middle of the power supply’s 2.5mm jack is not touching the 2.1mm pin in the device.

THIS fixes that. Convert a BIG 2.5mm plug jack into a smaller 2.1mm style that fits the majority of devices!

(FYI: Confused about “Jacks” and “Plugs”? Use a PLUG to fill a hole, being the jack/receptacle)