Couples who decide to live together without marrying need to talk through exactly what they expect from the relationship, says Janna Cordeiro, 30, of Atlanta. She is a nine-year veteran of the growing trend to cohabitation.

Increasing numbers of opposite-sex couples live together for reasons varying from convenience to practice for a scheduled marriage. The number of live-in, opposite-sex couples increased from 523,000 in 1970 to 4,236,000 in 1998, the Census Bureau says. Within about 18 months, most of those couples either marry or split up.

Just how to handle a live-in relationship is a mystery for many couples, experts say. "No widely recognized social blueprint or script exists for the appropriate behavior of the cohabitors themselves, nor for the behavior of the friends, family and other institutions with which they interact," say researchers Lynne Casper and Liana Sayer.