The covers enclose a cartridge extension mechanism capable of extending the pen cartridge from the housing upon application of a pressure to the respective covers, and retracting the pen cartridge into {HEAD ~ the housing} upon removal of {TAIL ~ the pressure} from the covers 
The covers enclose a cartridge extension mechanism capable of extending the pen cartridge from the housing upon application of a pressure to the respective covers , and retracting the pen cartridge into { HEAD ~ the housing } upon removal of { TAIL ~ the pressure } from the covers
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 OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH