How casually and carelessly most people go about the crucial task of selecting a partner. When you consider the emotional, or even the financial, consequences of mismatching, it is staggering to realize that such arrangements are frequently made by whim.

The accident of two people being available at the same time, the physical stirrings that accompany pleasant weather, an attractive look, a wellturned remark - even, Miss Manners blushes to say, the fact that one person has a convenient place to go: These are the reasons for which people commit themselves. They hardly bother to ask, or may be too delicate to volunteer, whether they are suited to each other in matters that count.

Then when they discover each other's inadequacies, it is too late to dissolve the relationship without complications and hurt feelings. That is how people find themselves committed to playing tennis with partners who make them miserable.

Miss Manners is not, herself, a tennis player. Although warned by her dear mother when she was a slip of a girl that "All nice men play tennis," she soon figured out that it it did not necessarily follow, therefore, that all nice men were seeking nice young ladies who played tennis. Nevertheless, Miss Manners is scrupulous about not limiting herself to giving advice in matters about which she happens to know something.

She does know that tennis players sometimes mistakenly feel that social rules prevail on the court when those of exercise or sports should apply. There is such a thing as purely social tennis, but this means, as in any other social activity, such as conversation, that one person does not seek to demolish the other, but both endeavor to keep the ball going back and forth. It is best, in such cases, not to keep score if one person is markedly superior to another.

Tennis for exercise also is better if inequalities between the players are minimized; it is better yet if people seek their equals. This is essential in tennis for sport. All social modesty and flattery should then be cast aside as people supply enough true information about their skills to enable them to find the right partners. If, as often happens in life, reality does not measure up to advance notice, the arrangements may be dissolved without hurt feelings, as never happens in life.

MISS MANNERS RESPONDS

A: "Oh, dear, I'm afraid I lost a diamond from my ring." You lose.

Q. In the opinion of etiquette, does it really matter, "manners wise," if a friend gives a baby shower for someone who is having her second child? Isn't a baby shower just a nice thing to do for a friend? I find it hard to believe there could be a "rule" saying only the first child receives a shower.

Q: I have very delicate skin, and have never even tried to get a suntan, after dreadful childhood troubles with burns. I now realize this is just as well, as I have been reading that sun can be damaging and drying to the skin. I have some summer hats to protect myself, but I am also considering using a parasol this year. When does one carry a parasol, and how?

A: Anyone who can muster the use of the umbrella can master the parasol, which is easier because it is not raining. That is, one keeps it close to the body when furled, and out of other people's faces when unfurled. One also takes care to do the furling and unfurling in an open space. The important thing to remember is that this instrument is not a weapon, as is, for instance, the walking stick.

Q: While walking through shopping crowds on a rainy Saturday afternoon, and while maneuvering through rush-hour crowds at subway stops on drizzly mornings, I have narrowly missed being speared by many an out-of-control umbrella. I assume that common sense should prevail, but just what are the rules for opening, closing, carrying and otherwise manipulating this potentially lethal accessory?

A: Managing an umbrella is just like managing a parasol, only it is raining out.

Q: My query concerns not only feeling incorrect, but feeling incorrectly, and more particularly, being felt incorrectly.

How do you feel about women who do (feel, that is) without invitation? Looking ahead to fall, I must soon decide whether to get my Norelco's heads refloated or to allow my beard to grow. But it is most disarming when women totalyy unknown to me reach out in a public place, grab my beard, and exclaim, "I just wanted to see what it feels like," or some such drivel. What is an appropriate response? Should such feelings be reciprocated?

A: Reciprocation is, of course, the ultimate social weapon. However, it could get you into trouble, which is one reason you might consider working for passage of the Equal Rights Amendment.