How to echo "$x_$y" in bash script?

view story

http://stackoverflow.com – It is very interesting that if you intend to display like "0_1" with bash using the code x=0 y=1 echo "$x_$y" then it will only display 1 I tried echo "$x\_$y" and it doesn't work. So my question is how to echo the form $x_$y ? I'm going to use it on a file name string. (HowTos)