Right now, there are 5 million heterosexual couples living together in the U.S. That's a 200 percent increase since 1980.

Many wonder how successful other couples are at cohabitating and how to handle various aspects of cohabitation.

Just in time, Dorian Solot and Marshall Miller have published something of a handbook for success. Unmarried to Each Other: The Essential Guide to

Living Together as an Unmarried Couple (Marlowe) answers questions they had themselves and gives answers to questions others ask them.

Most couples see living together as a stage between dating and marriage. With divorce being so common in our society, they want to make sure they make the right choice before marrying.

They are generally faithful to each other. Some 95 percent of the relationships are monogamous. The percentages of faithfulness for married and unmarried couples is about the same.

Those in longer-term relationships have to take steps to protect each other. Without a will, for example, a surviving partner would receive none of the deceased partner's property. Of course, without marriage, the remaining partner would receive no Social Security survivor's benefits.