This is a 4-pack of standard zither pins, used for tuning various stringed instruments. They have square heads and a serrated lower portion to help them grip a drilled hole. A wrench or tuning key is highly recommended (pliers will suffice, but aren't recommended). These can be used in place of tuners on traditional instruments, cigar box guitars and pretty much any other stringed creation.
These zither pins are considered to be the standard size: 1 5/8" (42mm) in length and .2" in diameter.