Specify frame by address

[Eli Zaretskii <eliz at gnu dot org>]

Both the doc string of the "frame" command and the manual say that it
accepts frame specification by address:

  (gdb) help frame
  Select and print a stack frame.
  With no argument, print the selected stack frame.  (See also "info frame").
  An argument specifies the frame to select.
  It can be a stack frame number or the address of the frame.  <<<<<<<<<<
  With argument, nothing is printed if input is coming from
  a command file or a user-defined command.

But what is "the address of the frame"?  I thought that using an
address of a function (the one shown by "info address") or some
address shown by the "backtrace" command would do, but none of them
seems to work.

What am I missing?

The context of this is that I would like to write a command that
automatically goes to a frame within a call to a specific function,
because that command needs to access variables local to that function.

TIA