There used to be a policy stating that anyone aged 55 and over could exclude up to $125,000 of his/her gains from the sale of a house. This was availed of under the premise that this is done only once.
The newer rules now are actually more specific. In one amendment, the age requirement was no longer in effect and the amount for exclusion was increased to $250,000 per person. Thus, a couple may claim tax deductions up to $500,000 from gains made on the sale of a house.
Later, the policy was revisited making the benefit available to anyone every two years. This means that every two years, anyone can sell a house and exclude up to $250,000 in gains from taxes.
Keep this in mind if you've sold a home this year, or planning to sell a home, but always check with your tax advisor to make sure the capital gains rules haven't changed again in a way that may negatively affect your situation if selling a home.