ISO 561:1989

The symbols have been selected in accordance with the following principles: a) the plants should be widely used; b) the symbols should be easy to draw and readily be distinguishable one from another; c) the principle and not the appearance of any particular type of machine should be represented, d) where a single symbol represents a group of items for which separate symbols are s-tandardized nationally, the ISO symbol should be readily distinguishable from any one of the national symbols.