What, exactly, is joint physical custody? Is it "fifty-fifty"? Is 33 percent enough? What about a 60-40 arrangement; or five nights out of 14? Is there some other "bright line," by which courts and litigants can readily see that a particular custodial arrangement is, or is not, joint physical custody? As Justice Donald King wrote some years ago in In re Marriage of Birnbaum (1989) 211 Cal.App.3d 1508:
It is doubtful that any two words mean as many different things to as many different people as the words "joint custody." 211 Cal.App.3d at 1515.
And it is equally doubtful that any two words have produced as much confusion in as short a time-span as those words applied in the context of cases in which one parent seeks to relocate with the parties' children.
Recently, in In re Marriage of Biallas (1998) 65 Cal.App.4th 755, an appellate court has told us what is not joint custody, for purposes of determining the ease with which one parent may relocate with the parties' child over the objection of the other. Biallas held that in a "move-away" case, a father who had his son in his care every other weekend from Friday evening until Monday morning, and one overnight each week, did not share joint physical custody. In so ruling, the Biallas court reversed a trial court ruling to the contrary.
Since 1980, when joint custody first became a statutorily-sanctioned form of custody order, knowledgeable family law practitioners have felt secure in advising clients that denominating a parenting plan as "joint physical custody" or "sole physical custody with rights of visitation" was less important than the actual custodial arrangement.
However, first in Birnbaum, supra, and more recently in In re Marriage of Burgess (1996) 13 Cal.4th 25, California courts have resurrected the significance of the de jure characterization of the custody order.
In Birnbaum, the appellate court addressed the issue of how a trial court is to determine whether the custodial arrangement is joint physical custody and whether joint custody requires a precisely equal division of custodial time. "Equal division of a child's time between the parents is not the hallmark of joint custody," the appellate court held [211 Cal.App.3d at 1515] -- the bright line, if there is one, is not at 50 percent.
Burgess, supra, raised the ante in the quest to quantify "joint physical custody." In Burgess, the California Supreme Court held that a parent having sole physical custody of a child has a presumptive right to relocate with that child absent a showing that the removal of the child would not be prejudicial to its rights or welfare. However, a different standard is imposed where the parties share joint physical custody. In now-famous footnote 12 of that opinion, the Burgess court wrote:
A different analysis may be required when parents share joint physical custody of the minor children under an existing order and in fact, and one parent seeks to relocate with the minor children. In such cases, the custody order "may be modified or terminated upon the petition of one or both parents or on the court’s own motion if it is shown that the best interest of the child requires modification or termination of the order." (Fam. Code, §3087.) The trial court must determine de novo what arrangement for primary custody is in the best interest of the minor children. [13 Cal.4th 40 n. 12]
Thus, if the parent wishing to relocate has joint physical custody, that parent's burden of proof is substantially higher than if he or she has sole physical custody.
Unfortunately, Burgess generated confusion over the meaning of "joint custody." The Burgess children had had almost daily contact with each of their parents during the initial period after separation, and thereafter, the children saw their father regularly and father had a "daily visitation routine," although the mother had sole physical custody. This arrangement could easily have been deemed joint physical custody.
In the 27 months between Burgess and Biallas, four appellate decisions have interpreted the meaning of "joint physical custody" in the context of Burgess's footnote 12 in various and contradictory ways. Two of those decisions resulted in reversals, two in affirmances; two of these decisions involved de novo review of joint custody arrangements, two involved sole custody.
The first two appellate decisions, Brody v. Kroll (1996) 45 Cal.App.4th 1732, and In re Marriage of Whealon (1997) 53 Cal.App.4th 132, were written by the same division of the Fourth District Court of Appeal. These decisions in particular exemplify the difficulty inherent in trying to define a custodial arrangement as "sole" or "joint."
In Brody, the paternity judgment provided that mother had primary physical custody of the child. Father spent Tuesday and Friday nights and all day Saturday with the child, except on those Saturdays when minor attended Hebrew school, plus "more extensive periods during summer." Actually, father saw the boy as frequently as four or five days a week. Mother sought to relocate with the childover father's objection, and the trial court granted her request.
The appellate court disagreed and reversed, ruling that on remand the trial court must determine custody de novo, stating that the parties had "an actual joint custody arrangement" both "under an existing order and in fact." In Whealon, the Court of Appeal affirmed a trial court's ruling that father was not sharing joint physical custody and that permitted the mother to relocate with the minor child. Mother had "primary physical custody" of the child; father had periods of custody every other weekend from Friday at 6:00 p.m. to Monday at 9:00 a.m. and one midweek overnight each week from Wednesday at 6:00 p.m. to Thursday at 9:00 a.m., plus certain time on holidays. For child support calculation purposes, this was held to be 28 percent. The court characterized this arrangement as "a case where one parent had, in substance, primary physical custody of the child and the other generous visitation rights." 53 Cal.App.4th at 142.
The custodial arrangement in Whealon was the same as that in Biallas. It appears that the aggregate amount of time which the father in Brody spent with his child was more frequent, but probably not more extensive, than the amount of fathers' custodial times in Biallas and Whealon . Yet the Brody court found that arrangement to be joint physical custody as a matter of law, the opposite conclusion as that reached by the Biallas court.
The two more recent of the pre-Biallas appellate decisions deepen the confusion.
In Ruisi v. Thieriot (1997), 53 Cal.App.4th 1197, the parties' dissolution judgment provided for "shared physical parenting." By stipulation, the father was with the boy Mondays from 3:30 p.m. to 7:00 p.m., Thursdays from 1:00 p.m. to 7:00 p.m., and Saturdays all day, although his time increased over the years. Despite the appellate court's statement that "it is not altogether clear hat under the 1986 agreement Kip and Paula truly shared joint physical custody," [53 Cal.App.4th at 1205], the time-sharing arrangement in Ruisi does not seem much different from that in Brody. Nevertheless, the former was deemed "joint" and the latter "sole."
Finally, in In re Marriage of Condon (1998) 62 Cal.App.4th 533, the court discussed a parenting arrangement which it described as falling "somewhere in between Brody and Whealon." In Condon, the court ordered joint legal and joint physical custody to the parties. By time of trial, father had custody on alternating weekends from Thursday afternoon until Monday morning, every Tuesday after school until 7:00 p.m., and on alternating Thursdays from after school until Friday morning. This custodial arrangement was close to equal time-sharing -- it does not appear to fall "somewhere in between Brody and Whealon."
Thus, Burgess and the five of its progeny that have addressed the issue of quantifying joint custody have left us with more questions than answers.
Although we now know, as a matter of law, that a parent who cares for the children every other weekend and one midweek overnight is not sharing joint physical custody, there are myriad custodial arrangements which could be characterized either way. And we have no appellate guidance whatsoever as to the impact of age and other factors on how a time-sharing plan will be characterized.
For example, custody plans for infants and younger toddlers frequently provide for one parent to have frequent, short periods of time with the child, perhaps four to eight hours, three to five times a week. Is this a joint custody arrangement, as in Brody, or a sole custody order as in Burgess and Ruisi?
What about the case of older children, who may be nominally in the custody of one parent, but spend the great majority of waking time at school, day care, and extra-curricular activities, and the balance of time (principally weekends) relatively equally between the parents? Similar considerations apply where the custodial parent spends long hours at work and, necessarily, away from the children.
And what weight is given to the child's vacation time, which often is spent primarily with the "non-custodial parent." If vacations are accorded equal weight with school periods, a non-custodial parent may actually have the children in his or her care close to half the year.
Counsel trying to negotiate this labyrinth may well look to Burgess for guidance, since the Supreme Court devoted a considerable portion of the opinion to emphasizing "the paramount need for continuity and stability in custody arrangements -- and the harm that may result from disruption of established patterns of care and emotional bonds with the primary caretaker." 13 Cal.4th at 32-33. In this and other aspects of the decision, the Supreme Court relied heavily on In re Marriage of Carney 1979) 24 Cal.3d 725, the seminal Supreme Court decision regarding changes of custody.
As did the Burgess court, the family law practitioner may return to Carney and note that Carney gave no consideration to toting up the seconds, minutes, hours and days that a child spends with one parent or the other. Rather, as Justice Stanley Mosk wrote for the Carney court:
[T]he essence of parenting is not to be found in the harried rounds of daily carpooling endemic to modern suburban life, or even in the doggedly dutiful acts of "togetherness" committed every weekend by well-meaning fathers and mothers across America. Rather, its essence lies in the ethical, emotional, and intellectual guidance the parent gives to the child throughout his formative years, and often beyond. [24 Cal.3d at 739]
An analysis focused on what each parent, respectively, brings to the children will, in the end, more thoroughly serve to illuminate the true nature of a parenting arrangement as joint or sole, than will reliance on the clock and calendar. Pending further appellate guidance, however, size (or at least, number of days) still matters.

From: http://www.starrecohn.com/articles/jointphy.html