Gnomon

The word gnomon has travelled in several interesting directions from its origins in the PIE root gno (to know) and Greek gignoskein (to come to know) and gnomonikos (someone who is able examine and judge). In brief, in Greek, a gnomon was ‘one who knows’.
 
How does a carpenter know if something is on the square? In ancient Greece, a carpenter’s rule or square was known as a gnomon. In Latin, a carpenter’s square was known as a norma, from gnomon.
 
How do you know the age of a horse or mule? By checking the gnomon of the animal; i.e., the teeth which indicate the age. “Their third and fourth teeth are called ‘gnomons’; that is ‘regulars’, because by them there is a tried rule to know their age” (Oxford English Dictionary, 1989).
 
How do you know what time it is? A gnomon is the vertical shaft or triangle that tells time by the shadow cast on your sundial. This use of gnomon comes to English in the 1540s from Latin and Greek gnomon.
 
What remains when you remove one of the corners of a parallelogram? A gnomon.
 
What about knowing or judging a person by the way they look? That is, as if judging a book by its cover? Such a judging of a person by their facial characteristics was known in the past as physiognomy—a now highly questionable approach to understanding human nature (i.e., judging or ‘knowing’ a person by their ‘physio’; i.e., by their ‘physical nature’.
 
What about the words ‘norm’ and ‘normal’? Yes, they are also rooted in Greek gnomon.
 
What about gnomes? Etymologically speaking, the words gnome and gnomon are cousins.
 
In brief, the word gnomon is related to many words related to knowing.
 
Reference: Online Etymological Dictionary, https://www.etymonline.com/
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