Can I install OS X on an external hard drive?

[amirite]

I'm running 10.11 and would like to install 10.10 on an external hard drive and boot from that.

How possible is this?

 

[chrfr]

I'm running 10.11 and would like to install 10.10 on an external hard drive and boot from that.

How possible is this?

If your computer can run 10.10, there's no problem doing this, but you will probably need to create a USB installer and boot from that because I don't believe the 10.10 installer will run in 10.11.