silly windows question

How do I choose windows system on my Yosemite Mac when restarting?

Believe it or not I couldn't find an answer using Google. ***? (is it such an easy problem that no one says anything about it?)

Hold down Option immediately after the chime and allm available drives/partitions will come up. Make your choice and away you go.

OK thanks. I had searched and not found an answer. I'll do that.

You can also, go into System Preferences, select Startup Disk, and select Windows or Boot Camp disk, then restart your Mac.

I bought two used Macbook Pros and one doesn't have Windows. Is that unusual?

Windows, as in Microsoft Windows? Macs, only come with one OS, usually, a version of Mac OS X/OS X/macOS.

Edit: If you want to use a different Operating System, you will need to purchase a copy of it to install it.