Feynman-like rules for constructing string vertices are formulated in the operator formalism in such a way that the underlyinh geometrical picture becomes manifest. Using Olive's as well as Lovelace's parametrization for the matter part we complete Feynman-like rules by including the ghost fields. The twisted propagator is defined with a b-ghost insertion together with a contour integration, in order to take the measure properly into account. The measure part of the string vertex is analyzed in detail. We formulate the BRST-invariant N-string tree vertex which enjoys the exact factorization property.
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