The hand is a flexible and convenient mnemonic device. The knuckle mnemonic tells us the number of days in a month, but a musical mnemomic called the Guidonian hand has been around for eight hundred years.
Most people have at least a hand (often two!). The Guidonian hand maps musical notes onto positions on the hand. A teacher could point to the joint of a finger and their student would know precisely what note to sing. Think “do re mi” but stretching for nearly three full octaves.
The lowest notes begin on the thumb, go down to the palm, up the pinky finger, and then circling around the other fingers until the highest notes fall at the centre of the middle finger.
It is named for Guido of Arezzo, a monk who invented the mnemonic and was also responsible for modern music notation (the stave / staff).
Oh, and the knuckle mnemonic (I knew it as “date palms”)? See the link below for that trick.