A line segment joining the midpoints of two sides of a triangle.

A triangle has 3 possible midsegments.

A triangle has 3 possible midsegments.

Try this Drag the orange dots on each vertex to reshape the triangle.
Notice the behavior of the midsegment line.

- The midsegment is always parallel to the third side of the triangle. In the figure above, drag any point around and convince yourself that this is always true.
- The midsegment is always half the length of the third side. In the figure above, drag point A around. Notice the midsegment length never changes because the side BC never changes.
- A triangle has three possible midsegments, depending on which pair of sides is initially joined.

See Constructing the midsegment of a triangle with ruler and straightedge.

- Triangle definition
- Hypotenuse
- Triangle interior angles
- Triangle exterior angles
- Triangle exterior angle theorem
- Pythagorean Theorem
- Proof of the Pythagorean Theorem
- Pythagorean triples
- Triangle circumcircle
- Triangle incircle
- Triangle medians
- Triangle altitudes
- Midsegment of a triangle
- Triangle inequality
- Side / angle relationship

- Perimeter of a triangle
- Area of a triangle
- Heron's formula
- Area of an equilateral triangle
- Area by the "side angle side" method
- Area of a triangle with fixed perimeter

- Right triangle
- Isosceles triangle
- Scalene triangle
- Equilateral triangle
- Equiangular triangle
- Obtuse triangle
- Acute triangle
- 3-4-5 triangle
- 30-60-90 triangle
- 45-45-90 triangle

- Incenter of a triangle
- Circumcenter of a triangle
- Centroid of a triangle
- Orthocenter of a triangle
- Euler line

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