It is, of course, absurd to say that any single tagline is the best ever written. After all, millions of slogans have been written, and there are no objective criteria by which we judge a tagline. We’re all entitled to our own opinions, as well.

But over the years, I’ve seen or heard thousands of advertising slogans, and through the lens of my copywriter’s eye, there is one tagline that stands above all others.

It’s only four words long, and one of the words is an article. But the four words work perfectly.

The line?

A diamond is forever.

What makes it so good? The line is very simple, yet filled with meaning. It literally means that a diamond can last for centuries. So this jewel can be a good investment.

Much more important is the line’s metaphoric meaning. It presents a diamond as a symbol of eternal love. By offering one of the most beautiful and indestructible objects on earth, a man or woman is promising love without end.

For many people, that’s the ultimate promise.

There is so much emotion packed into just a few words that the line is almost magical.

And what is most remarkable about the line is that it presents the product being sold, a diamond, as absolutely integral to the promise of undying love.

In addition to evoking an incredibly powerful emotional response, the line satisfies several other desirable tagline criteria.

The line is short, easy to say, easy to remember. The meaning is crystal clear—both the literal meaning and metaphoric meaning. There are no negative connotations.

The line is also unique and impossible to be duplicated by someone else. So it can be trademarked and protected.

Moreover, the line is incredibly flexible; it can work in any context. Since it’s a full sentence and names the product, it can stand on its own. But it also works in any kind of advertising.

The slogan was created as part of a De Beers advertising campaign in 1947, and I believe it has appeared in every one of the company’s engagement ring ads since 1948.

A female N.W. Ayer copywriter named Frances Gerety wrote the line. From 1943 to 1970, Ms. Gerety wrote virtually all of the copy for De Beers advertising. She passed away in 1999, two weeks after Advertising Age called “A diamond is forever.” the slogan of the twentieth century.

All in all, “A diamond is forever.” is as close to a perfect tagline as you’ll ever see.

It’s flawless.