Unions
is similar to a structure except that a structure reserves space for all of its members, a union only reserves spaces for its largest member
often implemented with a flag variable to tell the program how to interpret the variable
e.g. struct choice { int flags;
{ case (1) : puts(C.S.name); break;
case (2) : printf(“%d”,C.S.value; break;
default : printf(“bad flag value”); }