power supply - can laptop and its adapter be mixed and matched?

I was wondering if today's laptops and adapters together with power cords can be mixed and matched?

For example, my laptop is Lenovo T400 and its adapter outputs 20V and 4.5A. In my office, there is another laptop's adapter which outputs 19V and 4.3A. I was wondering if I forget to bring the adapter with my laptop to my office, will it be all right to use the other one instead?

What are some principles to tell if an adapter can be used by a laptop?