Can you use a card to open a knob lock?

The technical answer is NO.

But if a knob is not installed properly, then a card can just slide in an retract the latch. It is either the contractor don’t know about it (this is the majority case), or he just do a cincai work. Or maybe a lazy locksmith, because he certainly knows about the proper installation.

Next time you wana install locks, ask an opinion of a locksmith first.

www.finelockshop.com
“Advise from locksmith, not salesman”