Time magazine lists it as one of the 10 greatest speeches of all time. It is a poignant expression of the travails of a troubled nation. And it contains an opening line that most Americans can still recite years after they learned it in school.

The speech, of course, is Abraham Lincoln's Gettysburg Address. But although you know it well, what you might not realize about "four scores and seven years ago . . ." is that Lincoln's oration followed one of the most effective story structures you can use--the structure that storytelling expert Shawn Callahan calls "the clarity story."

This type of story is so valuable because for people to be engaged, they need to understand why they should take action. "The clarity story provides reasons in the most powerful and digestible format possible," writes Callahan in Putting Stories to Work.

Here's how Lincoln used the clarity story structure to build his famous speech:

Part 1 begins with a look back at the past to take the listener back to the way things used to be.

Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.

Part 2 shifts to something that happened: the events that caused a problem or opportunity.

Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure. We are met on a great battle-field of that war. We have come to dedicate a portion of that field, as a final resting place for those who here gave their lives that that nation might live. It is altogether fitting and proper that we should do this. 

Part 3 is what Callahan calls "so now . . ." which describes the decision or action needed to respond.

But, in a larger sense, we can not dedicate -- we can not consecrate -- we can not hallow -- this ground. The brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract. The world will little note, nor long remember what we say here, but it can never forget what they did here.

Part 4 looks ahead to the future to envision a desired outcome.

It is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. It is rather for us to be here dedicated to the great task remaining before us -- that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion -- that we here highly resolve that these dead shall not have died in vain -- that this nation, under God, shall have a new birth of freedom -- and that government of the people, by the people, for the people, shall not perish from the earth.

That's fine for Abraham Lincoln, but you may wonder: How can you use the clarity story for your own communication?

Callahan gives an example of a bank that adopted a new strategy of calling its branches "stores" as a way of emphasizing customer service. When a new CEO took over several years later, she decided to go back to using the old language of calling them "branches." Employees were confused about why the change was occurring when the bank had made such an investment in the move to "stores."

So the bank's leaders used the clarity story to communicate with employees:

(Part 1) In the past . . .  the bank wasn't delivering great customer service, so we made a number of changes, including referring to our branches as stores.

(Part 2) Then something happened . . . we began to hear from customers that they weren't comfortable with the language change; "stores" didn't seem serious enough.

(Part 3) So now . . .  we're changing back to referring to our branches as branches. We know the change will cost money, but we need to make sure we put our customers first.

(Part 4) In the future . . . we will continue to make changes that will increase customer satisfaction.

The structure works so well, writes Callahan, because it creates a series of events that cause people to want to know what happens next. "You need to spark people's interest by starting with the context, then hold their attention because something happens that causes a change, then end with an outcome."

Lincoln relied on this technique in his iconic speech--and you can, too.