On The Level Cufflinks

Am I standing up straight? Have I got a slight lean? Well now you can look at your sleeves for the answer

 Cufflink size : Medium

The spirit, or bubble, level, a sealed glass tube containing alcohol and an air bubble, was invented in 1661. It was first used on telescopes and later on surveying instruments, but it did not become a carpenter’s tool until the factory-made models were introduced in the mid-19th century. The circular level, in which a bubble floated under a circular glass to indicate level in all directions.

(Source http://www.britannica.com/EBchecked/topic/560417/spirit-level)