One Line

One Line is an edge-tracing puzzle. Start at a dot and travel along the drawing’s connections until every edge has been used exactly once. Revisiting a dot is allowed; retracing a used edge is not.

Rules & objective

Trace every edge of the drawing exactly once.

  • After choosing a start, each next dot must be connected to the current dot by an unused edge.
  • Dots can be visited more than once. A line crossing without a numbered dot is not an extra junction you can select.
  • Finish by using every edge. If you reach a dot with no unused exit too early, Undo lets you reconsider your route.

Key terms

Edge
A connection between two selectable dots.
Degree
The number of edges meeting at a dot.
Odd-degree dot
A dot with an odd number of connected edges.

Difficulty levels

One Line difficulty settings
LevelWhat changes
EasyShort paths with 3–5 edges. Three puzzles to explore.
MediumConnected loops and branches with 6–8 edges. Three puzzles.
HardDenser drawings with 9–12 edges. Three puzzles; Undo stays available.

Changing difficulty starts a fresh round. Personal bests are tracked separately.

This version on puzzle free online

Each difficulty contains three drawings. Easy uses 3–5 edges, Medium 6–8, and Hard 9–12. Next level cycles within the current tier; changing tiers returns to its first drawing.

Common questions

Must I hold the mouse down?

No. Select successive connected dots with taps or clicks.

Where should I start?

If the connected drawing has exactly two odd-degree dots, a complete trace must start at one and end at the other. If every dot has even degree, a complete trace can return to its start.