# Notation of ideals in ring theory

So right now i'm trying to solve this problem in ring theory for homework. The question pertains to proving that in a PID, D, if a,b are elements of D then the gcd of a and b can be written as a linear combination. In any event I know where I have to go but I'm stuck on this one bit of notation.

<a,b> = <d>.

I've never seen notation for an ideal like that with 2 elements separated by a comma. I'd appreciate any insight into this.

Oh and here's the site from wolfram where I originally discovered it:
http://mathworld.wolfram.com/PrincipalIdealDomain.html