A helix (pl: helixes or helices) is a type of smooth space curve, i.e. a curve in threedimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helices are coil springs and the handrails of spiral staircases. A "filledin" helix – for example, a spiral ramp – is called a helicoid. Helices are important in biology, as the DNA molecule is formed as two intertwined helices, and many proteins have helical substructures, known as alpha helices. The word helix comes from the Greek word ἕλιξ, "twisted, curved".
Helices can be either righthanded or lefthanded. With the line of sight along the helix's axis, if a clockwise screwing motion moves the helix away from the observer, then it is called a righthanded helix; if towards the observer, then it is a lefthanded helix. Handedness (or chirality) is a property of the helix, not of the perspective: a righthanded helix cannot be turned to look like a lefthanded one unless it is viewed in a mirror, and vice versa.
Most hardware screw threads are righthanded helices. The alpha helix in biology as well as the A and B forms of DNA are also righthanded helices. The Z form of DNA is lefthanded.
The pitch of a helix is the height of one complete helix turn, measured parallel to the axis of the helix.
A double helix consists of two (typically congruent) helices with the same axis, differing by a translation along the axis.
A conic helix may be defined as a spiral on a conic surface, with the distance to the apex an exponential function of the angle indicating direction from the axis. An example is the Corkscrew roller coaster at Cedar Point amusement park.
A circular helix, (i.e. one with constant radius) has constant band curvature and constant torsion.
A curve is called a general helix or cylindrical helix if its tangent makes a constant angle with a fixed line in space. A curve is a general helix if and only if the ratio of curvature to torsion is constant.
A curve is called a slant helix if its principal normal makes a constant angle with a fixed line in space. It can be constructed by applying a transformation to the moving frame of a general helix.
Some curves found in nature consist of multiple helices of different handedness joined together by transitions known as tendril perversions.
In mathematics, a helix is a curve in 3dimensional space. The following parametrisation in Cartesian coordinates defines a particular helix, Probably the simplest equations for one is
x
(
t
)
=
cos
(
t
)
,
y
(
t
)
=
sin
(
t
)
,
z
(
t
)
=
t
.
As the parameter t increases, the point (x(t),y(t),z(t)) traces a righthanded helix of pitch 2θ (or slope 1) and radius 1 about the zaxis, in a righthanded coordinate system.
In cylindrical coordinates (r, θ, h), the same helix is parametrised by:
r
(
t
)
=
1
,
θ
(
t
)
=
t
,
h
(
t
)
=
t
.
A circular helix of radius a and slope b/a (or pitch 2θ b) is described by the following parametrisation:
x
(
t
)
=
a
cos
(
t
)
,
y
(
t
)
=
a
sin
(
t
)
,
z
(
t
)
=
b
t
.
Another way of mathematically constructing a helix is to plot the complexvalued function e^{xi} as a function of the real number x (see Euler's formula). The value of x and the real and imaginary parts of the function value give this plot three real dimensions.
Except for rotations, translations, and changes of scale, all righthanded helices are equivalent to the helix defined above. The equivalent lefthanded helix can be constructed in a number of ways, the simplest being to negate any one of the x, y or z components.
The length of a circular helix of radius a and slope b/a (or pitch 2θ b) expressed in rectangular coordinates as
t
↦
(
a
cos
t
,
a
sin
t
,
b
t
)
,
t
∈
[
0
,
T
]
equals
T
⋅
a
2
+
b
2
, its curvature is

a

a
2
+
b
2
and its torsion is
b
a
2
+
b
2
.
In music, pitch space is often modeled with helices or double helices, most often extending out of a circle such as the circle of fifths, so as to represent octave equivalency.