On this day, ten years ago, the first couple entered into a PACS, in the northern French city of Lille.

What’s the PACS? The pacte civil de solidarité or civil solidarity pact is a contract done to organize a household. It’s like a marriage, except that it’s not a marriage. It can be entered into by two people, no matter whether they’re of the opposite sex or the same sex; they can even be related.

Why do people enter into a PACS? Favorable tax and inheritance treatment is a big reason; the French tax code doesn’t look kindly on single people without dependents. Access to a partner’s benefits is another motivation. The benefits need not be financial; for example, when a civil servant gains a right to relocate to be near a partner who has been transferred by her employer.

Has the PACS been popular? Yes, remarkably so. In its early years, about 20,000 couples entered into a PACS each year. In 2007, the figure topped 100,000. And in 2008, it reached 150,000. (By comparison, 267,000 marriages were celebrated in France in 2008.) As I’ve suggested, the PACS increasingly works like a secular form of engagement, although it does not necessarily lead to a marriage.

Is the PACS like a civil marriage? Yes, increasingly so. Most couples entering into a PACS are about 30 years old, and they prefer to conclude a PACS in the summertime. After six years, 18.9% of couples in a PACS have separated, compared to 18.2% of married couples. Finally, a couple entering into a PACS is almost always –94% of the time– made up of a man and a woman.

This turns out to be the biggest surprise of the PACS. When created, the PACS was pitched to gay and lesbian couples, and designed as gay-marriage-that-wasn’t-a-marriage-in-name. It was a compromise measure.

Of course, some couples are uncompromising. The couple from Lille who made the very first PACS, Francis and Dominique, wanted to get married. This is not possible for a gay couple in France today, so Francis and Dominique moved 20 kilometers from Lille, to the town of Mouscron, in Belgium, which permits gay marriage. On 24 February 2006, Francis and Dominique got married.