Abbreviated SI because in French it reads “Système International d’Unités” this is the standard set of basic units of measurement used around the world. Though the US is still hanging on to the Imperial (or British) System, which uses units of feet, inches, gallons, cups, etc., almost the entire rest of world communicates measurement through this metric system devised around the number ten. Be prepared for a constant headache if you are working concurrently on a couple of sets of construction drawings, and one is using SI and the other the Imperial System!