[vdupras]

Unlike most languages, Forth has two stacks. It sounds trivial, but it changes many things. It allows for a leaner call convention. With a single stack, every function call has to "shovel forward" its arguments over the function return address, where Forth "glides" through its arguments, making function calls significantly lighter.

[addaon]

> Unlike most languages, Forth has two stacks.

Like Forth, Ada has two stacks. Unlike Forth, which uses two stacks to simplify the language, Ada uses two stacks to complexify the language. This generalizes to other language features.

[kragen]

Ada's auxiliary stack is used to permit the returning of runtime-variable-sized objects from subroutines, which is also a thing you can use the operand stack for in most Forths.