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Cf. the first two sentences of 24:31 and the corresponding notes [37] and [38].
The specific, time-bound formulation of the above verse (evident in the reference to the wives and daughters of the Prophet), as well as the deliberate vagueness of the recommendation that women "should draw upon themselves some of their outer garments (min jalabibihinna)" when in public, makes it clear that this verse was not meant to be an injunction (hukm) in the general, timeless sense of this term but, rather, a moral guideline to be observed against the ever-changing background of time and social environment. This finding is reinforced by the concluding reference to God's forgiveness and grace.
He has forgiven what Muslim women did in the past because they were not obligated to cover up.
This is for all Muslim women, those of the Prophet's household, as well as the others. They were asked to cover themselves with outer garments when walking out of doors.
Jilbab, plural Jalabib: an outer garment; a long gown covering the whole body, or a cloak covering the neck and bosom.
The object was not to restrict the liberty of women, but to protect them from harm and molestation. In the East and in the West a distinctive public dress of some sort or another has always been a badge of honour or distinction, both among men and women. This can be traced back on the earliest civilisations. Assyrian Law in its palmiest days (say, 7th century B.C.), enjoined the veiling of married women and forbade the veiling of slaves and women of ill fame: see Cambridge Ancient History, 111. 107.
That is, if a Muslim woman sincerely tries to observe this rule, but owing to human weakness fails short of the ideal, then "Allah is Oft-Forgiving, Most Merciful".