--calcium and magnesium tend to be atagonistic in terms of action; they are both "alkaline metals" that tend to form ionic rather than covalent bonds.

So, if one takes both of them, there is a tendency for them to compete for the available "raw" hydroxol and other negatively charged bodily ions.

This is why many knowledgable authorities have told people to take both, and at different times of day, to minimize competition and maximize absorbability. I have seen the ratio of 2:1 calcium:magnesium quoted often--as long as one is taking more bioavailable forms such as citrates and asparates (as opposed to oxides).

Vitamin D, magnesium, and calcium all act to promote bone health; potassium and phosphorous are important players there, too. And sodium (another alkali metal), magnesium, and potassium are all important as electrolytic factors in the conduction of nerve impulses. The standard Western diet has been very imbalanced, however, in favor of too much sodium, and sometimes too much calcium--I am not convinced that a lot of people are calcium deficient--and too little magnesium and potassium (yes, there does seem to be some evidence, especially from countries where lactose intolerance is common and where overages of calcium are less likely, that the goodly intake of magnesium helps stave off osteopenia). And--vitamin D deficiency if also common, especially as one moves up the latitudes.