A tax unit is an individual, or a married couple who file a tax return jointly, along with all dependents of that individual or married couple. A tax unit is therefore different than a family or a household in certain situations. For example, two persons cohabiting would be considered one household but if they were not legally married, they would file separate tax returns and thus be considered two tax units. A family could consist of a married couple and the wife's elderly mother who lives with them. That family would be considered two tax units since, if the elderly mother had a large enough income, she would be required to file a federal income tax return on her own. Thus the number of tax units will tend to be larger than the number of families or households reported elsewhere.