C新手，求解一个问题

#include <stdio.h>
void main()
{
    char c;
    int letter=0,space=0,number=0,other=0;
    while ((c=getchar())!='\n')
    {
        if (65<=c<=90||97<=c<=122)//为什么无论输入什么字符，都会加到letter上！？
            letter++;
        else if(c=' ')
            space++;
        else if(48<=c<=57)
            number++;
        else 
            other++;
    }
    printf("%d\n%d\n%d\n%d\n",letter,space,number,other);
}