WHY??

#include<stdio.h>
union X
{
    int i;
    int j;
    double k;
}; // ; compulsory
struct Y
{
    int i;
    int j;
    double k;
};
int main()
{
    printf("sizeof X is %lu\n",sizeof(union X));
    union X x1;
    x1.i = 23;
    x1.j = 23;
    x1.i = 49;
    printf("%d %d\n",x1.i, x1.j);
    printf("sizeof Y is %lu\n",sizeof(struct Y));
    return 0;
}