How to set the executable bit on directories but not files?

For a long time, I was using contrieved methods to recursively change permissions. If you add the x bit, directories will be searchable, but all files will be marked as executables, which is probably not what you want. The magic is to use an uppercase X like so:

$ chmod -R u=rwX,g=rX,o=rX MYDIR

The X will instruct chmod to set the executable bit on directories and files that already have that bit set, but it will not set it on files that don’t have that bit set.