Capital losses can be used to offset capital gains, lowering your overall tax bill. As a rule, the losses must offset the gains made in the year when they are incurred. However, for many people, the gains and losses do not occur in the same year.
If that is the case, you may use capital losses to offset gains from the three previous years and indefinitely for future years. For example, if you sell a commercial building at a loss in 2016, you will be able to use the loss to offset capital gains going as far back as 2013 and indefinitely in the future. To apply losses to past years, use Form T1A (Request for Loss Carryback).