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This paper analyses losses caused by the misalignment of two fibers joined in a splice. We consider the possibility that the two fibers of different dimensions are separated in longitudinal direction and are tilted or offset with respect to each other. Central to our discussion is the observation that the modes of single-mode fibers are very nearly gaussian in shape regardless of the fiber type step-index or graded-index. The splice losses are thus related to the corresponding losses of gaussian beams. We specify the relation between the actual mode field and the gaussian beam that matches this field optimally. The trade-off between slice tolerances with respect to tilt and offset is expressed as an “uncertainty principle. “ Because of the near-gaussian nature of single-mode fiber fields, our results are immediately applicable to the excitation of single-mode fibers by gaussian-shaped laser beams.