We sometimes hear that time moves forward in one direction, like an arrow. History must too. Or perhaps it is an arc that, as they say, moves in one direction as well.

But American politics does not always move like an arrow, and the arc is sometimes frustratingly invisible. At those times, history is a pendulum, swinging.

That was a lesson the United States perhaps first learned in 1800, when control of the young government swung from the Federalist Party to the Democratic-Republicans, in the first modern peaceful transfer of power of its sort. Though that particular election took time to resolve, the change was sudden and drastic come Inauguration Day. The pendulum reached its limit, gravity did its thing, and in one day the rule of the nation switched over to those with a different and untested vision of how the world could be.

Such a change was built into the system. The men privy to the deliberations that first placed Thomas Jefferson in the White House would have remembered living under monarchy, a form of government that is defined by a straight line, that of heredity. The new democracy that they had built, with every protection against dynasty that they could think up, was meant to be different. And in the hundreds of years since, that process has generally continued, with political parties growing, dying and handing the reins to one another. That’s how a pendulum works: All along, as the mass goes as far as it can in one direction—even if that direction is a good direction—the energy is growing that could one day pull it to the other side.

In the years after the Civil War, the pendulum of American history was perhaps at its most obvious. (Many have seen this period’s echoes in recent events; for example, Slate’s Jamelle Bouie put the parallel in moving context early Tuesday morning.) In the wake of the Southern surrender at Appomattox, the federal government had to decide the conditions under which the former Confederate states would be able to reintegrate into the Union against which they had illegally revolted. Abraham Lincoln was murdered before he could see the country back to life; in his absence, the legislative and executive branches of the government disagreed about how best to accomplish what needed to be done. Some thought the seceding land should revert to federal ownership, starting statehood from scratch. Some, including President Johnson, preferred an easier route back to unity.

Under the lenient Reconstruction favored by Johnson, many former Confederate states established “black codes” constricting the rights of freedmen, attempting to hold onto as much of their old racial system as they could. Congress responded with tighter control of those states and legislation to protect the rights of former slaves, sparking a period of improvement that saw the election of the first black Senator and the building of thousands of schools for freedmen.

It lasted less than a decade.

In the 1870s, under federal complacency that set in after the passage of the 15th Amendment and without a strong hand from a distracted President Grant, the Democrats who supported white supremacy returned to power in the former Confederacy. The reaction to the progress that had been made was a violent retraction of the rights freedmen had gained. The pendulum had swung: War, Peace, Freedom, Constraint, Reconstruction, Reaction. As Robert Frost might put it, leaf subsides to leaf. The movement started before the war began and it kept going after the 1870s, but in that decade it was terrifyingly undeniable.

The other examples are many. The pendulum’s movement was clear for women in the 20th century: greater acceptance of working outside the home during World War II in the 1940s, a rejection of that progress under the family-centric conformity of the 1950s, the rise of feminism in the 1960s, the rise of the pro-life movement following Roe v. Wade in the 1970s.

Or see it crash through the economy, from the laissez-faire policies of the 1920s, to the federal involvement of the New Deal, to the so-called Roosevelt Recession of the late 1930s, to the strict controls of wartime oversight.

Or you could just watch the pendulum swing again in real time, from the vision represented by Barack Obama’s eight years in the White House to the wildly different future championed by President-elect Donald Trump.

But it’s also evident, thankfully, that the pendulum doesn’t always swing to the same extremes.

There are bad times in our history that we will not revisit, and good years that we hope in vain to pause. Just as a study of the past reveals how the narrative can so suddenly switch direction, it also clearly shows that we are not doomed to replay our worst moments indefinitely. The reaction is not always equal and opposite. Improvements to our laws, the evolution of our norms and the force of human goodness prevent that fate. Progress does happen. Falling short of synthesis does not mean that history literally repeats itself.

This truth is not just a matter of losing momentum. Instead, the pendulum stops and starts and changes direction.

Which, of course, a pendulum cannot do on its own.

We stand beside the pendulum, human. We are the ones who can reach out to grab the swinging mass of history. We must cradle it carefully, thoughtfully, before we decide to release it—because when we do, it will swing again. Swinging, we can only hope, is better than hanging still.

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