The trouble with Washington’s social scene is that it’s a horizontal city — and, try as one may, it’s hard to escape one’s assigned social stratum.

The lawyers know the lawyers; the journalists drink with the journalists; the lobbyists lobby the lobbyists (and the lobbyists-in-training, the Capitol Hill staffers).

By contrast, other cities are vertical. The most vertical city of all is New York. And Dublin is right behind it.

The test of horizontal versus vertical is in a city’s parties. If everyone at a party is either known to you or is in the same line of work — health care, information technology, law enforcement — you are living in a horizontal city; and you are trapped in a stratum that colors what you hear and ultimately think.

In a vertical city, a party is a wondrous place where you’ll learn things other than those things you think you know. A party that includes, say, an actor, a financier, a nurse, a tailor and a writer is a brew of delights.

In my experience, you’ll find such a party in New York above all other cities. However, there always seems to be an interesting mix at social gatherings in Ireland, and Dublin in particular. Generally, the Irish are so well read that they can talk to anyone about anything.

The legend goes that Washington parties are where things get done. If so, I’ve never been to those parties. I think in the days of Jack Kennedy, intimate, influential dinner parties — perhaps at the home of columnist Joe Alsop — were important.

Perle Mesta hosted parties that were as famous as they may have been influential (in Harry Truman’s day). Sally Quinn, wife of the storied Washington Post editor Ben Bradlee, has established herself as a Washington hostess with clout. And NPR’s Cokie Roberts is said to put on a great party.

But these are Washington parties for the like-minded and that’s the problem.

These days, I think the best party-giver in Washington is Gloria Dittus, a public relations phenomenon who recently sold her company but is still hosting parties at her Washington home. Just this week, she gave a grand bash for legendary White House reporter Helen Thomas and her new book, “Listen Up, Mr. President,” co-written with Craig Crawford.

Dittus knows how to throw a party.

First, she has a stunning home in the fashionable Kalorama section of Washington. But that’s just the beginning. She always has valet parking and coat-checking. And her parties are full bar and groaning board.

The party swirls from room to room in a natural flow. No bottlenecks at the bars.

Dittus knows who to invite across the social spectrum. Are great political deals cut at one of her parties? Maybe. But don’t blame her, or any hostess. Since bipartisanship has been replaced with the new Washington ethic of “my party right or wrong,” the deals are done intraparty in the depressing backrooms of Congress.

The other prevailing Washington value has reduced the party attendance: If you are in elected office, you’ll never be seen to take more than one glass of wine. Hardly worth dragging yourself across town for that.

Time was when at 6 p.m. the bottles came out all over Washington, and in the Capitol itself. Some of the best parties were impromptu ones in congressional offices. Nowadays, you have to settle for water; carbonated, if you’re lucky.

Then there’s the question of drinking and driving. Most of Washington’s denizens live in the suburbs and they have to drive. By contrast, parties thrive in New York and London, where there is more public transportation. But it should be noted that London parties tend to suffer from the same rigidities as Washington. Call it capital party syndrome.

Across the Atlantic, our democratic partners are still enjoying a drink. There are bars scattered all over the British Parliament, and two very busy bars in the much smaller Irish Parliament.

The principal virtue of partying is that it’s bipartisan. Until bipartisanship comes back, there is always the sainted Gloria Dittus. –For the Hearst-New York Times Syndicate