In classical mechanics, canonical coordinates are coordinates and in phase space that are used in the Hamiltonian formalism. The canonical coordinates satisfy the fundamental Poisson bracket relations:
A typical example of canonical coordinates is for to be the usual Cartesian coordinates, and to be the components of momentum. Hence in general, the coordinates are referred to as "conjugate momenta".
Canonical coordinates can be obtained from the generalized coordinates of the Lagrangian formalism by a Legendre transformation, or from another set of canonical coordinates by a canonical transformation.
Canonical coordinates are defined as a special set of coordinates on the cotangent bundle of a manifold. They are usually written as a set of or with the x's or q's denoting the coordinates on the underlying manifold and the p's denoting the conjugate momentum, which are 1-forms in the cotangent bundle at point q in the manifold.
A common definition of canonical coordinates is any set of coordinates on the cotangent bundle that allow the canonical one-form to be written in the form
up to a total differential. A change of coordinates that preserves this form is a canonical transformation; these are a special case of a symplectomorphism, which are essentially a change of coordinates on a symplectic manifold.
In the following exposition, we assume that the manifolds are real manifolds, so that cotangent vectors acting on tangent vectors produce real numbers.
Given a manifold Q, a vector field X on Q (a section of the tangent bundle TQ) can be thought of as a function acting on the cotangent bundle, by the duality between the tangent and cotangent spaces. That is, define a function
such that
holds for all cotangent vectors p in . Here, is a vector in , the tangent space to the manifold Q at point q. The function is called the momentum function corresponding to X.
In local coordinates, the vector field X at point q may be written as
where the are the coordinate frame on TQ. The conjugate momentum then has the expression
where the are defined as the momentum functions corresponding to the vectors :
The together with the together form a coordinate system on the cotangent bundle ; these coordinates are called the canonical coordinates.