Constants can be declared at the top level or inside other types. They must start with a capital letter:
PI = 3.14 module Earth RADIUS = 6_371_000 end PI #=> 3.14 Earth::RADIUS #=> 6_371_000
Although not enforced by the compiler, constants are usually named with all capital letters and underscores to separate words.
A constant definition can invoke methods and have complex logic:
TEN = begin a = 0 while a < 10 a += 1 end a end TEN #=> 10