JIMMY CARTER longed for a summit, figuring that if he could just go on one he could break through. But the Soviets made him wait more than two years until he had signed SALT II, and at the summit that followed there was no breakthrough, to say the least. Until the other day, Ronald Reagan had displayed no interest at all in a summit but, barely a month after Mr. Reagan entered the White House, Leonid Brezhnev has said he is ready to talk.

It is an instructive sequence. The American public has been authoritatively reminded that the Kremlin does not undertake the ascent to a summit simply on the basis of the measure of good will it may perceive in the White House. The Kremlin's calculus is more complicated. Presumably Mr. Brezhnev sees advantage in playing the man of experience, moderation and "realism" at a moment when a new American president is charging out in a hard and tough and -- to some -- alarming way. But it is evident that Mr. Reagan has the Politburo's attention. It sees merit in talking things over with him. It seems to have decided, furthermore, that he is not the sort who will "pay" for a summit. Hence the summit bid now.

The administration moved at once to accept it, as it should -- and not simply to prove it wasn't the laggard. The larger reason lies in the need to keep open a channel of communications that, for all the frustrations and misunderstandings it has produced over the years, retains not only political appeal but diplomatic potential.

Diplomats who have a certain vested interest in monopolizing diplomacy, traditionally advise that a summit must be "well prepared" -- and by them. That's good advice, and it appears Mr. Reagan will take it. But it also appears he has something else in mind. He wants to go into a summit feeling that, by the policy he has conducted up to them, he will have earned a certain Soviet respect. This is a worthy consideration, as long as it is not overdone. It could improve the prospects for a summit that would at the least reduce mutual misunderstandings -- that would be success enough. Finding new areas or even procedures of agreement would be gravy.

Summits are such catnip, to journalists at to politicians, that often it seems they are what the word "summit" suggests: "Gosh," FDR said on one occasion, "if I could only, myself, talk to some one man representing the Russians, I could straighten out the whole question." That's a good citation to put, as a cautionary note, on the first page of any summit manual.