@Opus 17 said "The parent where the child does live more than half the year is the only parent who can claim the child and they don't need a signed release from the other parent. The parent where the child does not live more than half the year can't claim them even if they were to get a signed release".
I think it goes like this: The parent where the child does live more than half the year is the only parent that can still claim the child under the "Qualifying Child" (QC) dependent rules. The parent where the child does not live (more than half the year) can only claim them under the "Qualifying Relative" (QR) rules and then only if the other parent agrees not to claim the child as a dependent (it does not have to be in writing). There are two significant differences. The QR dependent can not have more than $4700 of income and the parent (the child does not live) with must provide more than half the student's support (the QC rule is only that the student didn't provide more than half his own support).
The "special rule for children of divorced or separated parents" that come to an end when the child is "emancipated" (legally an adult) was the ability of the parents to "split the child" for tax purposes.
There is a special rule in the case of divorced & separated (including never married) parents. When the non-custodial parent is claiming the child as a dependent/exemption/child tax credit; the custodial parent is still allowed to claim the same child for Earned Income Credit, Head of Household filing status, and day care credit. This "splitting of the child" is not available to parents who lived together at any time during the last 6 months of the year; then only one of you can claim the child for any tax reasons. The tax benefits may not be split in any other manner.
