In mathematics, the **tautological one-form** is a special 1-form defined on the cotangent bundle *T***Q* of a manifold *Q*. The exterior derivative of this form defines a symplectic form giving *T***Q* the structure of a symplectic manifold. The tautological one-form plays an important role in relating the formalism of Hamiltonian mechanics and Lagrangian mechanics. The tautological one-form is sometimes also called the **Liouville one-form**, the **Poincaré one-form**, the **canonical one-form**, or the **symplectic potential**. A similar object is the canonical vector field on the tangent bundle. In algebraic geometry and complex geometry the term "canonical" is discouraged, due to confusion with the canonical class, and the term "tautological" is preferred, as in tautological bundle.

In canonical coordinates, the tautological one-form is given by

θ
=
∑
i
p
i
d
q
i
Equivalently, any coordinates on phase space which preserve this structure for the canonical one-form, up to a total differential (exact form), may be called canonical coordinates; transformations between different canonical coordinate systems are known as canonical transformations.

The **canonical symplectic form**, also known as the **Poincaré two-form**, is given by

ω
=
−
d
θ
=
∑
i
d
q
i
∧
d
p
i
The extension of this concept to general fibre bundles is known as the solder form.

The tautological 1-form can also be defined rather abstractly as a form on phase space. Let
Q
be a manifold and
M
=
T
∗
Q
be the cotangent bundle or phase space. Let

π
:
M
→
Q
be the canonical fiber bundle projection, and let

d
π
:
T
M
→
T
Q
be the induced tangent map. Let *m* be a point on *M*. Since *M* is the cotangent bundle, we can understand *m* to be a map of the tangent space at
q
=
π
(
m
)
:

m
:
T
q
Q
→
R
.

That is, we have that *m* is in the fiber of *q*. The tautological one-form
θ
m
at point *m* is then defined to be

θ
m
=
m
∘
d
π
.

It is a linear map

θ
m
:
T
m
M
→
R
and so

θ
:
M
→
T
∗
M
.

The tautological one-form is the unique horizontal one-form that "cancels" a pullback. That is, let

β
:
Q
→
T
∗
Q
be any 1-form on
Q
, and (considering it as a map from
Q
to
T
∗
Q
) let
β
∗
denote the operation of pulling back by
β
. Then

β
∗
θ
=
β
,

which can be most easily understood in terms of coordinates:

β
∗
θ
=
β
∗
(
∑
i
p
i
d
q
i
)
=
∑
i
β
∗
p
i
d
q
i
=
∑
i
β
i
d
q
i
=
β
.
So, by the commutation between the pull-back and the exterior derivative,

β
∗
ω
=
−
β
∗
d
θ
=
−
d
(
β
∗
θ
)
=
−
d
β
.

If *H* is a Hamiltonian on the cotangent bundle and
X
H
is its Hamiltonian flow, then the corresponding action *S* is given by

S
=
θ
(
X
H
)
.

In more prosaic terms, the Hamiltonian flow represents the classical trajectory of a mechanical system obeying the Hamilton-Jacobi equations of motion. The Hamiltonian flow is the integral of the Hamiltonian vector field, and so one writes, using traditional notation for action-angle variables:

S
(
E
)
=
∑
i
∮
p
i
d
q
i
with the integral understood to be taken over the manifold defined by holding the energy
E
constant:
H
=
E
=
c
o
n
s
t
.
.

If the manifold *Q* has a Riemannian or pseudo-Riemannian metric *g*, then corresponding definitions can be made in terms of generalized coordinates. Specifically, if we take the metric to be a map

g
:
T
Q
→
T
∗
Q
,

then define

Θ
=
g
∗
θ
and

Ω
=
−
d
Θ
=
g
∗
ω
In generalized coordinates
(
q
1
,
…
,
q
n
,
q
˙
1
,
…
,
q
˙
n
)
on *TQ*, one has

Θ
=
∑
i
j
g
i
j
q
˙
i
d
q
j
and

Ω
=
∑
i
j
g
i
j
d
q
i
∧
d
q
˙
j
+
∑
i
j
k
∂
g
i
j
∂
q
k
q
˙
i
d
q
j
∧
d
q
k
The metric allows one to define a unit-radius sphere in
T
∗
Q
. The canonical one-form restricted to this sphere forms a contact structure; the contact structure may be used to generate the geodesic flow for this metric.