  A new construction of biorthogonal splines for isogeometric mortar methods is
proposed. The biorthogonal basis has a local support and, at the same time,
optimal approximation properties, which yield optimal results with mortar
methods. We first present the univariate construction, which has an inherent
crosspoint modification. The multivariate construction is then based on a
tensor product for weighted integrals, whereby the important properties are
inherited from the univariate case. Numerical results including large
deformations confirm the optimality of the newly constructed biorthogonal
basis.
