Couples who separate permanently are still legally married. However long they are separated, they remain married until they are legally and finally divorced.
Sometimes a trial separation precedes a permanent one. During a trial time apart, the couple may see each other, try marriage counseling, and sound the beam and bulkhead of the ship of marriage to see if the vessel is sound enough to saved.
Even when couples agree to divorce, very often one or both spouses need time to adjust to the new situation. A permanent separation fills this gap. Sometimes couples who separate permanently do nothing more for some time.
As divorce lawyers will attest, in most marriages where a couple have made a good faith effort to "make the marriage work," it is a rarity to find two people who decide to divorce at the same time. One spouse calls it quits against the wishes of the other, and the person who is left needs time to catch up. For many, the permanent separation is the catchup time.
Very often the beginning of a permanent separation is the DOS -- date of separation, a very important date in affixing the responsibility for debts and the value of assets for distribution.
Some jurisdictions permit divorcing couples to life together even when divorcing; others require a physical separation -- separate residences. In a permanent separation, most couples who can live apart physically do so.
A permanent separation is sometimes placed ahead of a trial separation and below a legal separation. Each degree of separation, however, brings with it different legal parameters into play, and these may vary from one jurisdiction to another.
The act of separating, which is accompanied by announcements to family and friends that a divorce is in the offing, places a couple in a penumbra: the de facto end of marriage that still exists de jure. All manner of issues and problems can arise during this period, which, depending on the actions decisions of the spouses, lasts weeks, months and even years in some cases.
These issues and problems include, but are not limited to, assets acquired and the weight given them in property distribution; dramatic increases or decreases in assets; indebtedness incurred by one spouse during the separation; the one-sided childcare and homemaker efforts of parent; the use and occupancy of the marital home, including amortization of a mortgage.
In this, the phrase "during the marriage" comes into play because courts need a concept of the duration of the marriage for purposes of property division, particularly a date after which property is no longer subject to distribution. This date varies by jurisdiction. Some use the DOD, which is the date of divorce; some, the DOS, which is the date of permanent separation; others, the date of filing.
See also "During the Marriage."