During an interview with a person with visual impairment, I got to play with his nifty device called Victor Reader. Essentially, it’s a glorified MP3 player without a screen that dictates menu and content (audio books, ebooks, DAISY books, text files, notes, music, etc.). This particular model also records voice notes, but the newer model offers streaming radio over wifi and improved dictation.
Notice that all of the buttons are shaped differently based on their functions and some buttons even offer tactile or braille texture on the surface of the buttons.
Behavior of a button can go beyond physicality. For example, LG TV remote utilizes a subtle affordance that helps the user be aware of which button s/he has pressed without staring at the remote. The remote achieves this by having an unique pitch generated by each directional button. I discovered this when I pressed the buttons repeatedly and realized that the “click" sounds were slightly varied. In fact, up, down, left and right buttons all create slightly different pitches generated by the mechanical buttons/membranes. Furthermore, the volume and channel buttons offer slightly different contour that helps the user feel where their fingers are placed.
As you can see, the pitch that individual buttons generate is unique.