Many guitars, mandolins and ukuleles have a strap pin (strap button) which is also a jack socket, so you can plug your instrument into an amplifier or PA.   This combined pin and socket is a Bad Idea.  These endpins are larger than normal ones and often impossible to fit a strap onto.   Luckily, this adapter solves the problem! 

 

This simple device can be chosen as a free gift with any of Pinegrove guitar or mandolin straps.   It can also be ordered separately at https://www.etsy.com/uk/listing/547292600/guitar-strap-jack-endpin-adapter-to-fit