The master key should have a more squarish rubber handle on the top while the valet key will be roundish.

The owner's manual has a picture describing the difference. It's pretty obvious if you compare the two side by side.

The master key will unlock the glove box and the trunk. The valet key won't let you do that. That's really the only difference between the two.

You should get the steel copies made. I'm pretty sure using a brass copy leaves shavings in the ignition cylinder which can cause it to lock up.

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-Typos courtesy of my mobile phone.