Joint-custody means that two parents are equally responsible for taking care of a child. Both parents are considered the child's custodial parent.
If two parents of a child are married to or in a civil union with each other, they will have joint-custody.
Separated or unmarried parents can ask the court to give them joint-custody of a child. To do this, you will need to start a custody case. If you already have a custody order (parenting plan), you may be able to change (modify) it.