Book A Free Math Class

Types of Quadrilaterals: Definition and Classification

TL;DR

A quadrilateral is any four-sided closed shape whose interior angles sum to 360°, and the main types of quadrilaterals are the parallelogram, rectangle, square, rhombus, trapezoid, and kite. This article defines each type, shows how they nest in one family tree, and works through examples of classifying a shape from its sides, angles, and diagonals.

The One Word That Decides Which Quadrilateral You Are Holding

Show a class the shape [4, 4, 4, 4] and ask what it is, and half will say "square" and half will say "rhombus" — and both can be right at the same time. That overlap is not a mistake in the shapes; it is the whole point of how quadrilaterals are classified. The single feature that separates the six types is not how many sides they have (they all have four) but which sides are parallel and which angles are equal.

A quadrilateral is a closed, four-sided polygon, and its four interior angles always add to 360°. The named types of quadrilaterals — parallelogram, rectangle, square, rhombus, trapezoid, and kite — are sorted by three questions: how many pairs of sides are parallel, whether all sides are equal, and whether all angles are right angles. Because a shape can satisfy several of these at once, the types nest inside one another rather than sitting in separate boxes.

By the end you will be able to name any quadrilateral from its markings, explain why a square is also a rectangle and a rhombus, and read the family tree that ties all six together.

Types of Quadrilaterals: Why One Shape Can Have Many Names

The most useful way to hold the six types is not as a list but as a hierarchy. Reading top to bottom, each shape below inherits every property of the shape above it and then adds one more restriction. This is why a square is simultaneously a quadrilateral, a parallelogram, a rectangle, and a rhombus — each label describes a set it genuinely belongs to.

• Quadrilateral — four sides, angles sum to 360°. (The whole family.)
• Parallelogram — a quadrilateral with two pairs of parallel sides.
• Rectangle — a parallelogram with four right angles.
• Rhombus — a parallelogram with four equal sides.
• Square — a shape that is both a rectangle and a rhombus (four right angles and four equal sides).
• Trapezoid — a quadrilateral with exactly one pair of parallel sides (a separate branch, not a parallelogram).
• Kite — a quadrilateral with two pairs of adjacent equal sides and no parallel-side requirement (its own branch too).

A quick note on regions: quadrilaterals are also either convex (every interior angle under 180°, both diagonals inside) or concave (one reflex angle over 180°, one diagonal falls outside). All six named types above are convex; a "dart" is the concave cousin of the kite.

Each Type, Defined By Its Own Markings

Here is each type with the exact property that pins it down, plus the diagonal behavior that often gives it away.

Two terms defined on first use: opposite sides are the two sides that do not touch, and adjacent sides are two sides that share a vertex. The diagonals are the segments joining opposite corners. Watch the diagonals — they are the fastest fingerprint: equal diagonals point to a rectangle, perpendicular diagonals point to a rhombus or kite, and both at once point to a square.

Examples Of Types Of Quadrilaterals

Six worked classifications, from a single clear case to a shape you must reason about carefully.

Example 1

A shape has both pairs of opposite sides parallel and one interior angle of 90°. Name the most specific type it must be.
If one angle of a parallelogram is 90°, its opposite angle is also 90° (opposite angles equal), and the remaining two angles sum to 180° and are equal, so each is 90°. All four angles are 90°. Two pairs of parallel sides plus four right angles is the definition of a rectangle. It need not be a square, because the sides need not all be equal.

Example 2

Classify the quadrilateral with vertices A(0,0), B(4,0), C(4,3), D(0,3).
Wrong path first: a quick glance says "the sides look equal-ish, call it a square." Check the side lengths instead of guessing.
AB=4, BC=3, CD=4, DA=3. Opposite sides are equal (4,4 and 3,3) but adjacent sides differ (4≠3), so it is not a square. All angles are 90° (the sides run along the axes), so it is a rectangle, not a square. The lesson: never classify by appearance. Measure the sides and check the angles.

Example 3

A quadrilateral has all four sides equal to 6 cm but no right angle. Which type is it?
Four equal sides means it is either a square or a rhombus. A square requires four right angles; this shape has none. So it is a rhombus. A rhombus is a parallelogram (opposite sides are parallel), which is why its opposite angles are equal even though none is 90°.

Example 4

A four-sided shape has exactly one pair of parallel sides. Can it be a parallelogram?
No. A parallelogram requires two pairs of parallel sides. Exactly one pair of parallel sides is the definition of a trapezoid. The two parallel sides are the bases; the two non-parallel sides are the legs. If the legs are also equal in length, it is the special case of an isosceles trapezoid, whose base angles are equal.

Example 5

A kite has two pairs of adjacent equal sides: AB=AD=5 and CB=CD=8. Its diagonals are AC and BD. What is special about them?
In a kite, the two diagonals meet at a right angle. The diagonal connecting the vertices between unequal pairs (AC, the axis of symmetry) bisects the other diagonal BD. So AC⊥BD and AC cuts BD into two equal halves. This perpendicular-diagonal property is what a kite shares with a rhombus, but a kite has no parallel sides.

Example 6

True or false: every square is a rhombus, but not every rhombus is a square. Justify.
A square has four equal sides, which satisfies the rhombus definition, so every square is a rhombus — true. A rhombus has four equal sides but need not have right angles, so a rhombus is a square only when its angles are all 90°. Therefore not every rhombus is a square — also true.

Where the classification comes from: "one shape, sorted by symmetry"

Classifying quadrilaterals is not an arbitrary school exercise. It is a small instance of how mathematicians organize objects everywhere: sort by the symmetries and constraints each object satisfies, then let the sets nest.

• Historical root. Euclid's Elements already separated the "square, oblong (rectangle), rhombus, rhomboid (parallelogram), and trapezia," treating the more-constrained shapes as special cases of the looser ones. The modern family tree is a tidy version of that same idea.
• Why nesting matters. Because a square inherits every rectangle property, any theorem proved for rectangles (equal diagonals, for example) is automatically true for squares — you prove it once and get every special case free. This is the payoff of a hierarchy over a flat list.
• Where it goes next. The same "sort by constraints" logic scales up: it is how polygons split into regular versus irregular, how triangles split into scalene, isosceles, and equilateral, and how the whole of geometry organizes shapes by the transformations that leave them unchanged.

That destination — classification by symmetry — is why the family tree is worth learning as a structure, not six separate definitions to memorize.

Common Mistakes When Classifying Quadrilaterals

Mistake 1: Treating the types as mutually exclusive

Where it slips in: Being asked "is this a square or a rectangle?" and assuming only one answer can be right.
Don't do this: Say a square is "not a rectangle" because it "has a different name."
The correct way: A square is a rectangle (it has four right angles) and is a rhombus (it has four equal sides). The names are nested sets, not separate boxes. When a question asks for the type, give the most specific one that fits — "square" — while knowing the broader labels also apply.

Mistake 2: Classifying by how the shape looks instead of its measurements

Where it slips in: Judging a shape drawn on a page by eye.
Don't do this: Call a slightly-tilted rectangle a "square" because it "looks even," or call a shape a parallelogram because it "looks slanted."
The correct way: Classify only from the markings or measured values — parallel arrows, equal-side ticks, right-angle squares, or computed side lengths. The rusher who classifies from the picture will call a 4×3 rectangle a square every time; the fix is to always read the side lengths before naming the shape.

Mistake 3: Confusing "exactly one" with "at least one" pair of parallel sides for the trapezoid

Where it slips in: The trapezoid definition, where two conventions exist.
The correct way: Under the common school (exclusive) definition, a trapezoid has exactly one pair of parallel sides, so a parallelogram is not a trapezoid. Some textbooks use the inclusive definition ("at least one pair"), under which a parallelogram counts as a trapezoid. State which convention you are using, then be consistent.

Conclusion

• A quadrilateral is a four-sided closed shape whose angles sum to 360°; the six main types of quadrilaterals are parallelogram, rectangle, square, rhombus, trapezoid, and kite.
• The types nest in a family tree — each lower shape inherits the properties above it and adds one restriction.
• A square is both a rectangle and a rhombus; a rhombus is a parallelogram with four equal sides; a rectangle is a parallelogram with four right angles.
• The trapezoid (one pair of parallel sides) and the kite (two pairs of adjacent equal sides) sit on their own branches.
• Diagonals are a fast fingerprint: equal → rectangle, perpendicular → rhombus/kite, both → square.