Make a Comparative Timeline

A comparative timeline shows two or more sets of events against the same dates, so you can see what was happening in one place, life or project while something else was happening in another. A history class might set the Roman Empire beside the Han dynasty, a biography might set a writer's life beside the events of their era, and a case file might set one party's account beside the other's.

The idea is simple. What makes one readable is choosing the strands carefully and keeping them on one scale.

When a comparison helps

A comparative timeline answers questions that cut across strands: did this happen before or after that, what else was going on at the time, and how long did one stretch last next to another. If all of your events belong to one story, a single timeline with some grouping is enough, and How to Make a Chronology covers that.

1. Choose the strands

Two to four strands is usually the limit of what a reader can follow at once. Keep them the same kind of thing: two countries, two people, two versions of the same events. When one strand is much bigger than the others, such as a country set against a single person, the big one crowds the rest off the page.

2. Use one date range and one level of detail

Every strand shares the same axis, so the span has to cover all of them. Collect events at the same grain in each strand too. If one strand records every year and the other records only turning points, the dense one looks busier than it really was.

3. Look for the points where the strands meet

The payoff of a comparison is in the overlaps: a war that affects both countries, a law that changes both careers, a meeting both parties describe differently. Those are the entries worth checking twice, because they are the ones readers will look at first.

Building one in Preceden

In Preceden, each strand is a layer. Layers are stacked one above another on a single date axis, so events from different strands that happened at the same time line up.

Create a timeline, then click the Add Layer button above it to add a layer for each strand, named for what it holds, such as "Roman Empire" and "Han Dynasty". On a timeline made with the Simple layout, the empty starting layer, "Events", is removed when you add your first strand. The other layouts start with two or three layers called "Layer", which you can rename to your first strands by clicking their names in the list below the timeline.

When you add an event, choose its strand from the Layer menu. Give an event an end date when it covers a period, such as a reign or a job, and Preceden draws it as a bar, which makes overlapping stretches easy to see. Preceden also handles BC dates, so ancient comparisons work. Supported Date Formats lists what it reads.

Make the strands easy to tell apart

A few settings help readers keep the strands straight:

Give each layer its own color with the Color field when you add or edit it. Then click Appearance, open Colors, and check Override event color with the layer color so every event takes its strand's color.
Under Appearance > Layers, set Layer Name Position to Left of the layer so each strand's name runs down the side of its events.
In the same section, check Automatically compress layers to fit more events on each row. Each layer then needs fewer rows, so the strands sit closer together and are easier to compare.
Reorder the layers so the strands you compare most often sit next to each other.

To set a strand aside while you work on the others, collapse its layer. Exports and shared links still show every layer.

If the strands are already separate timelines

If you built each strand as its own timeline, you can merge them into one by downloading one timeline as a spreadsheet and importing it into the other. Imported events go into the layer with the same name, and Preceden creates any layer that is missing. Give the two timelines' layers different names before you merge, or both strands will land in the same layer. Importing a spreadsheet requires a paid plan.

Comparing stretches from different eras

Sometimes the comparison is by elapsed time rather than by calendar date, such as the first five years of two companies founded decades apart. Preceden places every event on a real date, so for this kind of comparison use stand-in dates instead. Start each strand at the same stand-in year, such as 2000 for year one, and follow Dateless Timelines to hide the stand-in years from readers.

When it is ready, export it as a PDF for a handout or report, or share a link so others can explore it online.