This plural family of one father, three mothers and 21 children lives in Salt Lake Valley among monogamous families.

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The institution of marriage is a constant, practiced in one form or another all over the world. In most places, marriage is between a man and a woman. Some states or countries allow for marriage between a woman and a woman or a man and a man.

In general, people seem to be comfortable with the two-person marriage. You can find it everywhere. Plural marriage, on the other hand, also known as polygamy, is legal nowhere in North America, underscoring the depth of conviction held by the select few who practice it anyway.

At the mention of polygamy, many minds go immediately to Muslims and Mormons. In fact, mainstream Mormons (the Church of Jesus Christ of Latter Day Saints, or LDS) don't practice polygamy; some fundamentalist break-off sects, no longer part of the LDS, do practice it. And Fundamentalist Mormons and Muslims aren't the only ones practicing polygamy. Some Neopagans, including Wiccans, are open to the practice, as are several Liberal Christian groups. And way back in the Bible, Abraham, the first Jew, and his grandson Jacob are reported to have had more than one wife.

Most polygamists in North America, though, are practicing plural marriage as the Divine Principle, a component of the Mormon belief system. It was made very public, perhaps for the first time, in the HBO series "Big Love." The Divine Principle is still on the books in the LDS Church, but its practice has been discontinued. Mormons who refused to give it up were ultimately excommunicated from the LDS.

Estimates have their numbers at somewhere between 30,000 and 50,000 in North America, most living in small communities in parts of the American West.

Polygamy is something of a mystery to most people living outside those communities. Outsiders are typically offered only a partial glimpse, and then it's in the context of a "raid" like the one that resulted in the arrest of Prophet Warren Jeffs, the removal of hundreds of children from their multiple mothers, and those rare photos of rural women wearing identical prairie dresses.

This is not, of course, the full picture of Fundamentalist Mormon polygamy. It's barely a sketch. For one thing, what's the Principle really about? When did it begin, how has it been practiced, and how, if plural marriage is illegal, are tens of thousands of people marrying more than one spouse?

It starts with a man named Joseph Smith, who had a revelation.