They assembled in Arlington: the blonde with the blonde, the bouffant with the bouffant, the bald with the bald. Thelma with Velma in red and white matching outfits. Edith and Edna (77) the oldest twins present.

The 8-week-old couplet (fraternal); the pretty duet of psychologists (identical, mirror image); the logistics engineer (alone, without his twin) who believes he's the "third kind" (one egg infiltrated by two sperm).

Five hundred multiple births, 250 sets, it was speculated, gathered at Stouffer's Hotel in Arlington for the 43rd Annual International Twins convention, and since most of them were identical you heard the usual twin tales, that is the tales twins love to tell"singletons" (which is what those of us who neglected to share a womb are called) about date-swapping and teacher-fooling and exam-switch-and twin-talk.

But none of them had anything on The Richmond Four.

The Richmond Four are two sets of identicals who married each other and live in the same house in Aurora III.

"We call ourselves 'The Richmond Four,'" explained LaVona Richmond, twin to LaVelda, "and we sign all our letters. 'Twincerely Yours, The Richmond Four.'"

And they've dressed alike, "Ever since we were knee-high to a grasshopper." said Alwin Richmond, brother to Arthur. "We used to fight when we weren't dressed alike." "Because," said LaVona, married to Alwin, "we feel we're an extension of each other. We feel it's the greatest honor on earth to be a twin. And because . . ."

Here LaVelda, married to Arthur, broke in and they said it in unison, "Not everyone can be a twin and not everyone can be an identical twin."

"And our husbands proposed together," continued LaVona Richmond, "they both went down on their knees, two months after we started dating."

And the ladies accepted together, because they had both planned on doing that. And then they had a double ceremony together. "Thirty sets of twins attended our wedding," recalled LaVona, "Our organists were twins, our flower girls were twins."

And the older twins wound up with each other. LaVona and LaVelda, like their husbands, were born six minutes apart. And when LaVelda had appendicitis, LaVona said she felt the pain of the disease - and then the cutting of the surgeon's knife. And during their wedding rehearsals, the minister accidentally married the wrong pair. And once, one of the men accidentally kissed the wrong twin, but as LaVona said, "They don't kiss alike."

And yes, they would just love to have twins, themselves. And they all met because of the initiative of LaVona and LaVelda who spotted the dress-alike-look-alikes on the streets of Aurora, and then called to ask them to come to an International Twins Convention.

So this is all very romantic and we are back where we started. There are now a number of people at the convention who seem slightly uncomfortable with the idea of The Richmond Four. Not the marriage so much - there is, after all, yet another married set among the membership, but they don't live together or dress alike - just the totality of their involvement with each other.

Twins - especially in the numbers that turned out for this convention - are a disconcerting, almost frightening vision to non-twins. The identical ones are born without the delusion most of us wish upon ourselves; that we are unique in every way, that no one really looks like us, acts likes us or possesses those disarming, distinguishing sensibilities and talents that are ours alone.

Of course, twins themselves are forever pointing our they are not precisely identical either. But the fact is they often look that way. The fact is as the book "Gemini" points out, a number of societies throughout history have been spooked by the arrival of human duplicates.

"Killing one or both twins seemed an ideal solution for several Brazilian and American Indian tribes, who believed that birthing a single child was human, but birthing a litter - inhuman or bestial."

"Singletons," says Donald Keith of Chicago, a twin himself, "don't understand what we accept. I accept it . . . I wouldn't trade being a twin for anything on earth. But I would not want to be my brother, nor he me . . . When we were kids, of course, we loved to put on the same clothes and spook the world."

A number of twins seem to feel that way: that they are something apart from the rest of the world. And yet Keith and his twin didn't get along very well until they were 28. After that, they became very close. "Before that - we each were trying to mold the other in our own image." A number of twins at the convention remarked on their closeness: that they felt they probably would die about the same time; that they were "overwhelmed" by another's Labor pains; that they knew when a twin was missing or in trouble.

Most of the identicals at the convention were dressed alike. But Dr. Janet W. Kizzier and Dr. Judy W. Hagedorn, identical twin psychologists who were co-presidents of the International Twins Assn., have some strong opinions on that. They were not dressed alike. On the other hand they both wore tan, and seemed somewhat annoyed by the coincidence.

"We went into his organization to change its image," said Hagedorn, "and we have not been tremendously successful, because as you can see, they're dressing alike. We believe in choice."

"Well, I didn't go into this organization to change its image," objected Kizziar, grinning, "We joined to sell books."

It is the twins who have written "Gemini" which informs the reader that "twins occur approximately one in 80 human births . . . About one-third of all twins are identical."

Judy Hagedorn wants her organization to hold seminars, get involved in research and genetic studies. Well, that sounds fine, except there are several in the membership who aren't so crazy about her ideas. LaVona Richmond, for instance, who says, "We are not in agreement with their book because it's negative toward twins. And we stress the positive."

And Beverly (twin name: Bebe Simmons, for another instance, who says, "We're not encouraging medical (research) within is wrong with us, we'll go to a doctor."

Summons, as it happens, is the identical double of Elspeth (twin name: Beth) Corley; co-leaders of the Atlanta branch and co-coordinators of the internaional group. Yesterday the twins were elected co-presidents of the organization.

The ITA does everything in "co's." So do Simmon/Corley - these days, anyway. For the past six years, they lived together in the same house and both mother Corley's daughter. They run the same catering firm. They wear the same clothes for business. They twin-talked as infants, shared the same radio program as teen-agers. They were also both turning 47 yesterday.

"We joined the ITA," said Corley with a chuckle, "because my daughter wanted to see if other twins are as crazy as we are."

Well, the Corley-Simmons twins are anything but crazy. But Elspeth Corley does have a way of explaining their lives by starting in the middle, and jumping around from there.

And her twin has a way of saying, very consolingly, "Just relax, Honey. You'll understand us eventually."

Actually, for identicals, they're quite different from each other.

"I'm more impulsive," said Simmons, "and she thinks things out more. That's it."

"I'm more logical," agreed corley.

"Oh, she drives me crazy," laughed Simmons.

"I never understood why my (late) husband picked me,"said Corley, who was divorced before he died. "I could never see why, he picked me over Beverly. Because she's such an outgoing person."

"Beth," said Simmons, "is quite reserved. Beth is quite sensitive and a very gentle kind of person."

The twins have known tragedy lately. In the last seven years their parents died, their brother died; Corley's daughter was in a bad car accident, from which she is now recovered. That is why Simmons now says, "Let's say in the last seven years we got too close."

They've had their faces X-rayed and the structure is exactly alike, except for a fractional variation in the bridges of their noses. They wear the same curly wigs to cover their identical pony tails.

In school they came up with the same answers to exam questions, and - as Beth Corley says - we were accused of cheating. But even when we were separated, we came up with the same answers. And Bebe never studied."

But the major characteristic that separates them now is their voices. Beth Corley's is light and fluid. Bebe Simmon's is throaty and duskier.

"That's intentional," said Simmons, "and also because I smoke."

They changed from identical aquapantsuits to identical bags, and descended to the convention room in the hotel. On the way they met Thelma and Velma who were dressed exactly alike, and looked exactly alike. One of them said, "Even when we don't plan to, we often end up wearing the same clothes."

Thelma and Velma, Bebe and Beth, LaVona and LaVeda, Arthur and Alwin - all of them gathered with hundreds of those similarly born, similarly fated and similar to watch the contest. Eddie Tigner, leader of the Ink Spots and blessed, it is claimed, with two sets of twins himself, was one of the judges.

And they sat in rapt attention as the categories for the contest were called out:

Most alike "Infants in Arms" . . .Most Alike Ladies . . . Most Alike Men . . . Most Unalike Females . . . Most Alike Girls . . . Most Alike Boys . . .

"Congratulations." whispered LaVona Richmond to the McCammon twins (Pat and Pam) who are identically beautiful and won one set. Then she resumed her narratice about her life.

It seems that LaVona Richmond, like LaVelda Richmond, has gained 25 pounds since their double marriage (vows said in unison) one year andthree months ago.