Qualifying Child
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Per IRS Publication 17 Your Federal Income Tax, page 27:

Qualifying Child

Five tests must be met for a child to be your qualifying child. These five tests are:

  1. Relationship,
  2. Age,
  3. Residency,
  4. Support, and
  5. Joint return.

CAUTION! If a child meets the five tests to be the qualifying child of more than one person, a special rule applies to determine which person can actually treat the child as a qualifying child. See Qualifying Child of More Than One Person, later.