Is it permissible for a woman who wears the niqab to wear a single-layered niqab, or must she wear a multi-layered niqab with a string between the eyes to ensure the concealment of the face when the wind blows, or when eating and drinking in front of people, or during any sudden movement?

A woman is religiously obligated to cover her face. This can be done with any garment that conceals her face, as long as it achieves the primary purpose of covering. It is not necessary to have a thread in the middle or to wear two layers if one layer provides sufficient covering. The proof for this is that a woman in a state of ihram (definition) for Hajj or Umrah (definition) is not permitted to wear a niqab, as the Messenger of Allah, peace and blessings be upon him, said: "The woman in ihram should not wear a niqab, nor should she wear gloves," narrated by Al-Bukhari. If she fears the gaze of foreign men, she should draw something down over her face.