Copy a segment, step by step
The first construction anyone learns.
Copy AB onto the ray from C.
Copy AB onto the ray from C.
- Copy AB onto the ray from C. Two tools only — a compass that holds a distance and a straightedge that draws a line. Nothing here is measured.
- 1. Open the compass so its point is on A and its pencil is on B. Opening the compass to A and B stores the length. Nothing is measured and nothing is written down — the distance itself is now held between the two points of the tool.
- 2. Keeping that opening, put the point on C and draw a short arc crossing the ray. Moving the compass does not change what it holds, so the arc you swing from C is exactly |AB| away from C, all the way round.
- 3. Label the crossing D. CD is the same length as AB. Where that arc meets the ray is the only point on the ray at distance |AB| from C. That is what makes D exact rather than close.
- Leave every arc showing. The arcs are the evidence the figure was constructed rather than eyeballed, and a marker reads them to check the method as well as the result.
How it works.
- 01
Copy AB onto the ray from C. Two tools only — a compass that holds a distance and a straightedge that draws a line. Nothing here is measured.
- 02
1. Open the compass so its point is on A and its pencil is on B. Opening the compass to A and B stores the length. Nothing is measured and nothing is written down — the distance itself is now held between the two points of the tool.
- 03
2. Keeping that opening, put the point on C and draw a short arc crossing the ray. Moving the compass does not change what it holds, so the arc you swing from C is exactly |AB| away from C, all the way round.
- 04
3. Label the crossing D. CD is the same length as AB. Where that arc meets the ray is the only point on the ray at distance |AB| from C. That is what makes D exact rather than close.
- 05
Leave every arc showing. The arcs are the evidence the figure was constructed rather than eyeballed, and a marker reads them to check the method as well as the result.