What is the ruling if the compared limb is one whose viewing is permissible?

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If a man compares his wife to a part of his mother whose viewing is otherwise unlawful, such as the vulva or the thigh, it constitutes Zihar, according to Abu Hanifa. However, if the compared limb is one whose viewing is not unlawful, such as the head, it does not constitute Zihar, by the reasoning of Abu Hanifa, as it is a comparison to a part whose viewing is permissible, similar to comparing her to a limb of another one of his wives.