Who, What, or Where is this?

• I only work when you're sitting down.

• I can be put together in a snap.

• My length is adjustable.

• Despite my name, I don't hold up your chair.

• I always sit in your lap when I'm at work.

• I am now available for dogs as well as people.

• My image is illuminated in airplanes.

• I can save lives.

• Most cars have at least four of me.

• U. S. law requires use of me.

Think you know the answer?

Answer:  tleb taes  (read from right to left)