A controller, consisting in {TAIL ~ part} of a single panel or a series of panels, is provided to selectively open and close the apertures in response to the orientation of {HEAD ~ the eraser} as it contacts the chalkboard 
A controller , consisting in { TAIL ~ part } of a single panel or a series of panels , is provided to selectively open and close the apertures in response to the orientation of { HEAD ~ the eraser } as it contacts the chalkboard
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH