Capital Gains Tax on a second home

[Todd Bright]

I have a question for anyone familiar with the tax laws involved with capital gains tax.

My father in-law has been scrambling to get his second home ready for market to sell this year because he believes he will have to pay capital gains tax. From my understanding, you have to pay capital gains tax when you sell the house, not from holding it. 

More info to clarify, this house was his original primary residence until he bought a second home in 2013. He now lives in the new home he bought so it's considered his primary. 

Thanks for any help in advance.