Mono Monday: Four

I spent quite a while this morning racking my brains to think of a suitable shot for a Mono Monday theme of 'Mineral World' until I looked at Blip and realised everyone else was doing 'Four'; I'd mixed up the themes with 'Tiny Tuesday'. So here is my Four; the tension adjusters on one side of a Mandolin neck.

Amazing to think Mono Monday is four years old; it must be one of the most enduring challenges on Blip. Many thanks to Skeena for this week's challenge, and to him and all the other hosts that have kept it going all this time.