The millimetre (American spelling millimeter) is a unit of length and is a multiple of the SI unit metre. It uses the symbol mm.
There are 1000 mm in 1 m and 10 mm in 1 cm. From this one can derive that there are 0.03937 in.
The millimetre is sometimes verbally abbreviated to 'mil' - which can cause confusion in the US as there is a unit there that means thousandth of an inch.
The ell is a unit of length and originates from historic England and uses the symbol ell.
The ell usually measured 45in or 1.143m but was origally a "cubit" - which was the length from your elbow to the tip of your fingers and literally translated from the word "arm". It was mostly used in the tailoring world when buying and selling textiles.
Although now obsolete, the word "ell" still survives in the English word "elbow".