We define a Poisson Algebra called the swapping algebra using the intersection of curves in the disk. We interpret a subalgebra of the fraction algebra of the swapping algebra – called the algebra of multifractions – as an algebra of functions on the space of cross ratios and thus as an algebra of functions on the Hitchin component as well as on the space of SL n (R)-opers with trivial holonomy. We relate this Poisson algebra to the Atiyah– Bott–Goldman symplectic structure and to the Drinfel'd–Sokolov reduction. We also prove an extension of Wolpert formula.