As someone with a fair amount of experience with driving on snow and ice (I lived in Maine and central New York for a combined 20 years), I cringe whenever I see quotes such as the following, taken from recent posts:
From a Phoenix resident:
Not much snow is around these parts. Are snow chains recommended (there)?
From a Dallas native:
I will be leaving tomorrow. I will go ahead and purchase chains.
From an Angeleno:
Would we need chains for snow at all?
The first thing you have to understand is that chains are not some magic accessory that will let you drive through any weather or road conditions that you might encounter no matter what your level of expertise. Since they are only on the rear wheels, they help not at all in controlling the direction your car is heading. And they are simply not capable of pushing your car through snow that rises above its front bumper.
Chains have their place, and in the right conditions with an experienced driver can make the difference between getting through and getting bogged down, which is why they are required on certain roads at certain times. But simply buying a pair, even if you practice putting them on in ideal conditions before you go, will not guarantee that you will be able to complete a trip. Please, if you have little or no experience with winter driving, seriously consider how much you need to or want to make the journey that you are setting out on, and read up on both winter driving and, if appropriate, mountain driving before venturing out into conditions that can and do get people killed.