This is a repeat, but may interest newcomers:
Pen and Feather are varieties of the same word, the root being the Sanskrit pat, to fly. (We have the
Sanskrit pattra, a wing or instrument for flying; Latin, petna or, penna, pen; Greek, pteron; Teutonic,
phathra; Anglo-Saxon, fether; our “feather.”)
Analogous examples are TEAR and LARME, NAG and EQUUS, WIG and PERUKE, HEART and COEUR,
etc.