<p>There has been significant developments in the classification of boundary conditions of positive symmetric systems, also known as Friedrichs systems, after the introduction of operator theoretic framework. We take a step forward towards applying the abstract theory to the classical framework by studying Friedrichs systems on an interval. Dealing with some difficulties related to the smoothness of eigenvectors, here we present an explicit expression for the dimensions of the kernels of Friedrichs operators only in terms of the values of the coefficients at the end-points of the interval. In particular, this allows for a characterisation of all admissible boundary conditions, i.e. those leading to bijective realisations.</p>