An apparatus includes an overlay mark. The overlay mark includes a first portion including a first pattern and a second portion including a second pattern. The first pattern includes a plurality of first features and a first center feature each separated by a gap along a first direction. At least two gaps are the same but are different from the other gaps, and the first centers are symmetric with respect to the first center feature. The second pattern includes a plurality of second features and a second center feature each separated by a gap along the first direction. At least two gaps are the same but are different from the other gaps, and the second features are symmetric with respect to the second center feature. The first center feature of the first pattern is aligned with the second center feature of the second pattern along a second direction.