Liouville class of a Lagrangian submanifold ($[\lambda {|}_L]$)

For a Lagrangian submanifold $L\subset ({\mathbb{R}}^{2n},d\lambda )$, the restriction $\lambda {|}_L$ is closed. Its cohomology class is preserved by ambient symplectomorphisms because every closed one-form on ${\mathbb{R}}^{2n}$ is exact. On a product torus of circles, its periods are the enclosed symplectic areas of the factors.