To claim a personal exemption, the taxpayer must be able to answer "no" to the intake question, "Can anyone claim you or your spouse on their tax return?"
This applies even if another person does not actually claim the taxpayer as a dependent. Taxpayers who could be claimed as a dependent must claim "0" exemptions. This means they will not be able to subtract the exemption amount from their gross income, and they may have to use a smaller standard deduction amount. See the lesson Standard Deduction and Tax Computation for more information on this topic.