Systematic error. The zero error of the screw micrometer is caused by the fact that the zero scale line of the movable scale does not coincide with the zero scale line of the fixed scale when the two anvils are close together, which belongs to the system error. The solution is to adjust the fine-tuning screw at one end of the movable scale so that the edge of the movable scale is 0 degrees relative to the fixed scale when the two measuring anvils are aligned.