Triangle Vocabulary Worksheet

Press Print worksheet to match twelve triangle terms, from scalene and isosceles to hypotenuse, to their meanings.

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NameDateMade at homeroomtools.comDraw a line from each one on the left to its partner on the right.Triangle vocabularytrianglescalene triangleisosceles triangleequilateral triangleacute triangleright triangleobtuse trianglehypotenuselegbasebase anglevertex anglea triangle in which no side islonger than anothera triangle that has a pair of equalsidesthe side that does not touch the90-degree cornera triangle whose angles are eachunder 90 degreeseither angle at the ends of thebottom sidea side that touches the 90-degreecornera triangle with one angle ofexactly 90 degreesthe side a triangle is drawnstanding onthe angle where the two equalsides meeta triangle with one angle greaterthan 90 degreesa triangle whose three sides alldiffer in lengtha closed figure made of threestraight segments
NameDateMade at homeroomtools.comDraw a line from each one on the left to its partner on the right.Triangle vocabularyAnswer keytrianglescalene triangleisosceles triangleequilateral triangleacute triangleright triangleobtuse trianglehypotenuselegbasebase anglevertex anglea triangle in which no side islonger than anothera triangle that has a pair of equalsidesthe side that does not touch the90-degree cornera triangle whose angles are eachunder 90 degreeseither angle at the ends of thebottom sidea side that touches the 90-degreecornera triangle with one angle ofexactly 90 degreesthe side a triangle is drawnstanding onthe angle where the two equalsides meeta triangle with one angle greaterthan 90 degreesa triangle whose three sides alldiffer in lengtha closed figure made of threestraight segments
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All 36 terms

Kinds and sides 12

  • acute triangle A triangle whose angles are each under 90 degrees. Angles of 55°, 60° and 65° make an acute triangle.
  • base The side a triangle is drawn standing on. Any side of a triangle may be chosen as its base.
  • base angle Either angle at the ends of the bottom side. The two base angles of an isosceles triangle are equal.
  • equilateral triangle A triangle in which no side is longer than another. Each angle of an equilateral triangle measures 60°.
  • hypotenuse The side that does not touch the 90-degree corner. No other side of a right triangle is as long as the hypotenuse.
  • isosceles triangle A triangle that has a pair of equal sides. A triangle with sides 5, 5 and 8 is isosceles.
  • leg A side that touches the 90-degree corner. In a right triangle with sides 5, 12 and 13, the legs measure 5 and 12.
  • obtuse triangle A triangle with one angle greater than 90 degrees. Angles of 20°, 40° and 120° make an obtuse triangle.
  • right triangle A triangle with one angle of exactly 90 degrees. Angles of 30°, 60° and 90° make a right triangle.
  • scalene triangle A triangle whose three sides all differ in length. A triangle with sides 4, 5 and 6 is scalene.
  • triangle A closed figure made of three straight segments. Its three angles add up to 180°.
  • vertex angle The angle where the two equal sides meet. In an isosceles triangle the vertex angle is opposite the base.
Show all 36 terms (24 more)

Lines and centers 12

  • altitude A segment from a corner, perpendicular to the far side. Every triangle has three altitudes.
  • angle bisector A ray that splits an angle exactly in half. Bisecting an angle of 70° leaves two angles of 35°.
  • centroid The meeting place of the medians. A cardboard triangle balances on a pencil tip at its centroid.
  • circumcenter The center of the circle through all three corners. The circumcenter is the same distance from each corner.
  • concurrent Said of lines that all cross at a single spot. The three altitudes of a triangle are concurrent.
  • Euler line A line through the centroid and the orthocenter. The circumcenter lies on the Euler line too.
  • exterior angle The angle outside a triangle made by lengthening a side. It is as large as the two far interior angles put together.
  • incenter The meeting place of the angle bisectors. The incenter is the center of the circle that fits inside the triangle.
  • median A segment joining a corner to the far side's midpoint. A median splits a triangle into two parts of equal area.
  • orthocenter The meeting place of the altitudes. In an obtuse triangle the orthocenter lies outside the triangle.
  • semiperimeter Half of the distance around a figure. A triangle with sides 3, 4 and 5 has a semiperimeter of 6.
  • vertex Any one of the three corners of a triangle. Triangle ABC has vertices A, B and C.

Congruence and theorems 12

  • congruent The same in shape and in size. Two triangles with sides 3, 4 and 5 are congruent.
  • corresponding In the matching position in another figure. In similar triangles, corresponding angles are equal.
  • cosine For an acute angle: neighboring leg over hypotenuse. In a 3-4-5 right triangle, the cosine of the angle next to 4 is 4/5.
  • Heron's formula A rule that gives the area from the three side lengths. For sides 3, 4 and 5 it gives √(6 × 3 × 2 × 1) = 6.
  • proportion An equation that sets one ratio equal to another. 3/4 = 6/8 is a proportion.
  • Pythagorean theorem Leg squared plus leg squared equals hypotenuse squared. With legs 3 and 4: 9 + 16 = 25, so the hypotenuse is 5.
  • Pythagorean triple Three whole numbers that fit as right-triangle sides. 5, 12 and 13 form a Pythagorean triple: 25 + 144 = 169.
  • similar Alike in every angle, with sides in one fixed ratio. A triangle with sides 3, 4, 5 is similar to one with sides 6, 8, 10.
  • sine For an acute angle: facing leg over hypotenuse. In a 3-4-5 right triangle, the sine of the angle opposite 3 is 3/5.
  • theorem A result shown to be true by a chain of reasoning. A theorem is proved from statements already known to be true.
  • triangle inequality Any two sides together are longer than the third. Sides of 2, 3 and 6 cannot form a triangle, since 2 + 3 is less than 6.
  • trigonometry The study of how a triangle's sides and angles relate. Sine and cosine are part of trigonometry.

How this list was compiled: Term list chosen for this site on 2026-10-03. Each meaning is written in our own words and was checked that day against the entry for the term in Wiktionary, the free dictionary (https://en.wiktionary.org/), whose text is under the Creative Commons Attribution-ShareAlike license. The terms were chosen with the articles Triangle and Isosceles triangle of English Wikipedia open, read the same day (https://en.wikipedia.org/; Creative Commons Attribution-ShareAlike license). The figures in the examples are our own working. The example after each meaning is our own. Vertex angle and base angle, in their sense for an isosceles triangle, follow the article Isosceles triangle, and the triangle inequality follows the article Triangle inequality of English Wikipedia; both were read on 2026-10-05, when every meaning was read against its source again. Licence: own work

How it works

This triangle vocabulary worksheet prints twelve terms down the left and their meanings, mixed up, down the right. Three sets cover the kinds of triangle and their sides, the lines and centers, and congruence and theorems.

Read the full guide

Three sets

Set opens on Kinds and sides: scalene, isosceles, equilateral, acute, right and obtuse triangle, hypotenuse, leg, base, base angle and vertex angle. Lines and centers adds altitude, median, angle bisector, centroid, orthocenter, incenter, circumcenter and Euler line.

Congruence and theorems

The third set prints congruent, similar, corresponding, Pythagorean theorem, Pythagorean triple, triangle inequality, Heron's formula, trigonometry, sine, cosine, proportion and theorem. Under the tool each term has a worked example, such as: 5, 12 and 13 form a Pythagorean triple.

  • The sheet prints words only. No triangle is drawn on it.

Questions and answers

What are the six types of triangles?

By sides: scalene (no two sides equal), isosceles (two sides equal) and equilateral (three sides equal). By angles: acute (every angle under 90 degrees), right (one angle of 90 degrees) and obtuse (one angle over 90 degrees).

What is the difference between a median and an altitude?

A median joins a corner to the middle of the opposite side. An altitude runs from a corner to the opposite side and meets it at a right angle. The medians cross at the centroid and the altitudes at the orthocenter.

What are the four centers of a triangle?

The centroid, where the medians cross; the orthocenter, where the altitudes cross; the incenter, where the angle bisectors cross; and the circumcenter, the center of the circle through the three corners.

Is an answer key printed?

Yes, as a second page: the worksheet again, with a line from each term to the meaning that belongs to it. The check box for it is under More options.

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