This is one of the most common structures in Western
ideography. The signs , , , and are used with different meanings
in some 30 well-known ideographic systems from the Bronze Age to the
present day, and in many alphabetical writing systems. The structure
is especially common in modern-day technical and scientific systems
(mainly because it is a letter in the Greek alphabet). One should not
forget the use of as a sign for the number
The sign has been used to represent fire in alchemy and a modern day reflection of this use is the logotype of McDonald's. In some modern military contexts it indicates air transportation, and in art it is a common (pictorial) sign for a flying bird.
Turned upside down, , it is used in some meteorological systems as a sign for certain types of high, thin clouds.