I need to copy a formula down and have it increase the column letter. For example

=A$1 and copy that down so it goes to
=B$1
=C$1 and so on and only increase the letter and not the number. I know a person can do this horizontaly but I need this copy procedure to work vertically.

This looks so easy but I can not find the answer anywhere. Any help would be greatly appreciated.

Thanks.