Assertion: concertina's button | /r/AtLocation | front face
Facets:
Source sentences:
text: As the buttons are on the front face and cannot be seen by the player, an aid to navigation is provided by a small depression, hole or bump on the central C button in the root row, often supplemented by similar or different tactile marks on other selected root-bass buttons, such as the A♭ and E, which are four buttons away in either direction.