In elementary geometry, two are perpendicular if they intersection at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the perpendicular symbol, ⟂. It can be defined between two lines (or two line segments), between a line and a plane, and between two planes.
Perpendicularity is one particular instance of the more general mathematical concept of orthogonality; perpendicularity is the orthogonality of classical geometric objects. Thus, in advanced mathematics, the word "perpendicular" is sometimes used to describe much more complicated geometric orthogonality conditions, such as that between a surface and its normal vector.
Perpendicularity easily extends to Line segment and rays. For example, a line segment $\backslash overline\{AB\}$ is perpendicular to a line segment $\backslash overline\{CD\}$ if, when each is extended in both directions to form an infinite line, these two resulting lines are perpendicular in the sense above. In symbols, $\backslash overline\{AB\}\; \backslash perp\; \backslash overline\{CD\}$ means line segment AB is perpendicular to line segment CD.
A line is said to be perpendicular to a plane if it is perpendicular to every line in the plane that it intersects. This definition depends on the definition of perpendicularity between lines.
Two planes in space are said to be perpendicular if the dihedral angle at which they meet is a right angle.
More precisely, let be a point and a line. If is the point of intersection of and the unique line through that is perpendicular to , then is called the foot of this perpendicular through .
To prove that the PQ is perpendicular to AB, use the SSS congruence theorem for QPA' and QPB' to conclude that angles OPA' and OPB' are equal. Then use the SAS congruence theorem for triangles OPA' and OPB' to conclude that angles POA and POB are equal.
To make the perpendicular to the line g at or through the point P using Thales's theorem, see the animation at right.
The Pythagorean theorem can be used as the basis of methods of constructing right angles. For example, by counting links, three pieces of chain can be made with lengths in the ratio 3:4:5. These can be laid out to form a triangle, which will have a right angle opposite its longest side. This method is useful for laying out gardens and fields, where the dimensions are large, and great accuracy is not needed. The chains can be used repeatedly whenever required.
In the figure at the right, all of the orangeshaded angles are congruent to each other and all of the greenshaded angles are congruent to each other, because vertical angles are congruent and alternate interior angles formed by a transversal cutting parallel lines are congruent. Therefore, if lines a and b are parallel, any of the following conclusions leads to all of the others:
For another method, let the two linear functions be: and . The lines will be perpendicular if and only if . This method is simplified from the dot product (or, more generally, the inner product) of Euclidean vector. In particular, two vectors are considered orthogonal if their inner product is zero.
A line segment through a circle's center bisecting a chord is perpendicular to the chord.
If the intersection of any two perpendicular chords divides one chord into lengths a and b and divides the other chord into lengths c and d, then equals the square of the diameter.Posamentier and Salkind, Challenging Problems in Geometry, Dover, 2nd edition, 1996: pp. 104–105, #4–23.
The sum of the squared lengths of any two perpendicular chords intersecting at a given point is the same as that of any other two perpendicular chords intersecting at the same point, and is given by 8 r^{2} – 4 p^{2} (where r is the circle's radius and p is the distance from the center point to the point of intersection). College Mathematics Journal 29(4), September 1998, p. 331, problem 635.
Thales' theorem states that two lines both through the same point on a circle but going through opposite endpoints of a diameter are perpendicular. This is equivalent to saying that any diameter of a circle subtends a right angle at any point on the circle, except the two endpoints of the diameter.
The major axis of an ellipse is perpendicular to the directrix and to each latus rectum.
From a point on the tangent line to a parabola's vertex, the other tangent line to the parabola is perpendicular to the line from that point through the parabola's focus.
The orthoptic property of a parabola is that If two tangents to the parabola are perpendicular to each other, then they intersect on the directrix. Conversely, two tangents which intersect on the directrix are perpendicular. This implies that, seen from any point on its directrix, any parabola subtends a right angle.
The product of the perpendicular distances from a point P on a hyperbola or on its conjugate hyperbola to the asymptotes is a constant independent of the location of P.
A rectangular hyperbola has that are perpendicular to each other. It has an eccentricity equal to $\backslash sqrt\{2\}.$
The altitudes of a triangle are perpendicular to their respective bases. The perpendicular bisectors of the sides also play a prominent role in triangle geometry.
The Euler line of an isosceles triangle is perpendicular to the triangle's base.
The DrozFarny line theorem concerns a property of two perpendicular lines intersecting at a triangle's orthocenter.
Harcourt's theorem concerns the relationship of line segments through a vertex and perpendicular to any line tangent to the triangle's incircle.
Each of the four of a quadrilateral is a perpendicular to a side through the midpoint of the opposite side.
An orthodiagonal quadrilateral is a quadrilateral whose are perpendicular. These include the square, the rhombus, and the kite. By Brahmagupta's theorem, in an orthodiagonal quadrilateral that is also cyclic, a line through the midpoint of one side and through the intersection point of the diagonals is perpendicular to the opposite side.
By van Aubel's theorem, if squares are constructed externally on the sides of a quadrilateral, the line segments connecting the centers of opposite squares are perpendicular and equal in length.

