A Gantt chart is a set of horizontal bars against a calendar. It is the default way of showing a project plan, and it comes in two quite different forms that look identical on paper.
One is a drawing: bars where you put them. The other is a model: bars connected by dependencies, so that moving one moves everything downstream automatically.
Why the arrows matter
Without dependencies, a chart is a snapshot. Testing slips by a week, you drag the bar, and every task that depended on testing is now wrong — silently, because nothing told you.
With dependencies, the software knows that deployment cannot start until testing finishes. Move testing and deployment moves with it, and the end date updates. That recalculation is the entire value of project management software, and it is why a spreadsheet of bars is not a substitute for one on a project with real interdependencies.
What follows from dependencies is the critical path — the chain of tasks where any delay delays the whole project. Tasks off that path have slack and can slip without consequence. You cannot see either from bars alone.
When bars alone are the right tool
When you are communicating rather than planning. A proposal, a status update for people who will not touch the plan, a printed timeline for a wall.
For those the arrows add clutter and nothing else, because nobody is going to reschedule anything from the picture. What matters is that it is legible.
Where the chart came from
Henry Gantt popularised it around 1910, and it was used to plan the Hoover Dam and the American interstate system. Karol Adamiecki had published the same idea in Poland fifteen years earlier as the harmonogram, in Polish and Russian, which is why the name that stuck is not his.
Every chart was drawn by hand, and redrawn from scratch whenever anything moved. That constraint explains a good deal about how rarely plans changed in that era, and it is why the arrival of software that recalculated was such a change in practice rather than just in convenience.
A detail that matters more than it sounds
Both ends of a task should be inclusive. A task from the 5th to the 5th is one day, not zero.
Get this wrong and single-day tasks — a launch, a review, a handover, a go-live — render as zero-width bars, which is to say invisible. Those are among the most important things on any plan.
Questions people ask
Should every project have a Gantt chart? No. For work that arrives continuously rather than in a planned sequence, a board is a better fit. Gantt suits projects with a defined end.
How detailed should the tasks be? Coarse enough to fit on one page. A chart with sixty rows is not read.
What about milestones? Conventionally a diamond rather than a bar, since a milestone has no duration.
Is a Gantt chart the same as a roadmap? No. A roadmap shows themes over quarters and is deliberately vague about dates. A Gantt chart commits to them.
The Gantt chart maker turns a task list into a printable timeline, and the business day calculator, date range overlap and quarter date range handle the arithmetic behind the dates.