As more and more parties to a divorce with children both seek to have a large role in their children's lives, many parties are now either agreeing by consent to shared physical custody or a party seeks shared physical custody.
In the past, the norm may have been that one parent was the stay at home or worked part time or was the primary nurturer. More and more often today, both parties wish to have a primacy of nurturing in their children's lives. Therefore, rather than, in terms of custody, one parent is the parent of primary residence and one parent is the parent of alternate residence, many parties are now agreeing upon shared physical custody.
The old norm often had the mother as the parent of primary residence and the father having the children for overnights on alternate weekends.