Certain - definite, sure, reliable necessary - essential, required, needed operation - functioning, performance, activity mountings - fixtures, fittings, supports to protect - safeguard, shield, defend to fit - install, fix, attach auxiliary - supplementary, additional, secondary injecting - infusing, introducing, administering to enable - allow, facilitate, empower to position - place, locate, arrange to connect - link, join, attach to supply - provide, furnish, give device - gadget, tool, apparatus rate - speed, pace, frequency air vent - air duct, ventilation, air passage to insert - place, insert, embed