Most GM's had rounded keys for the doors and truck,glove box,and a square ended one for the ignition too...but I have found in several instances,I was able to start a vehicle with a door key,and vice versa--the "letter code" on the keys had to match up most of the time..(GM has several alphabetical codes stamped on the key just before the toothed area like an "A" or "B","G","K","L" etc...)..
A friend who runs a repair garage spent 125 bucks for a set of "master keys" to unlock vehicles doors and start them...when he got them,he was pretty disgusted--they were simple stamped sheet metal "blanks" with teeth on them...looked like something you'd buy at Toy's R Us for your toddler to play with!..
He was ready to send them back,when his co-worker said "what the hell--lets TRY them and see if they DO work"...they went outside to a 2004 Ford Expidition that a customer left to be repaired,and LOCKED the keys in it,hidden under the floor mat!..

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He stuck a key in that had the double sided teeth like some other Ford keys from other vehicles in the lot had,and he fumbled with it for several minutes-no go,he was getting irritated...then another guy pulls in,who runs a body shop a few miles away,and he sees us fiddling with the lock,and he comes over and says "here--let ME show you how to use these keys--I know your thinking they SUCK,but they DO work"!..
He jiggled the key up and down rapidly,while trying to turn it,and sure enough,within about 10 seconds,the door lock pops up,and he opened the door!..after practicing with those keys,my friend has had pretty good luck opening doors with them...but on ignitions,they are no help on most vehicles made the past 20 years,that have "chipped" keys,because even if you can get the ignition to turn,its still not gonna start...but he's used them to get the ignition cylinder out,so he can replace it in a few vehicles..usually he has to call a locksmith on newer vehicles because they have to mach the chip to the computer--a few cars had to be towed to a dealer and have the computer re-flashed for it to start and run...400+ bucks..
