The evolution of the Qu'ranic jilbab to what can be worn today has to follow some simple guidelines. It must a. be an overgarment (i.e something worn over one's regular clothing), b. loose fitting, c. not see-through, d. not be decorated in an ostentatious manner that increases one's social status, e. not resemble men's clothing, and f. not confuse people about whether you are affiliated with something that is only for the disbelievers such as a haraam behaviour or shirk. A simplification of the Qu'ranic jilbab (albeit a more colourful version) worn by the Prophet's wives (R.A) is the Saudi milhafah overhead abaya that may be cut to only reveal one eye or may be cut with a space for the niqab and is often open in style. It is an overhead abayaat that is cut like a farasha. If closed it is an Egyptian style referred to as Isdal. Only very recently did this type of abayaat evolve to have pronounced t-shaped sleeves. That is because the original garments worn by the Ansaar were more like floor length khimars/and Iranian chador clothes. Anyways, from this style is derived the the popular farasha (butterfly) shoulder cut abaya, and the thobe and bisht abayaat. The modesty of the overhead abayaat have recently been popularized again in the farasha, thobe, and bisht shoulder designs.The shoulder abayaat fell out of the scene a little ways when the shoulder/robe abaya came on the scene in the late 80s. Heading towards the nineties there was harsh debate whether it was appropriate for young Saudi ladies, lol, especially when the skin tight french cut became popular. When skin tight became a trend, the abaya stopped being proper jilbab. With 9/11 a renewed interest in preserving the name of Islam helped many sisters research their hijabs to better explain it to prejudiced persons and more modest loose fitting robe abayaat began to return, as well as more Western takes on the Qu'ranic classic, such as open styles (only hijab when also paired with a modest long dress underneath that is itself an overgarment not the undergarment).Before the open abaya ever hit the scene though, places like Turkey and Syria were manufacturing what are termed "jilbabs"----long modest loose-fitting floor length coats that opened, buttoned and zipped. In the West, sisters found it very hard to find long sleeved floor length dresses that were loose fitting, so many started wearing a long coat with a long loose skirt, and long loose tunic tops with long loose skirts, making the traditional hijab that conisted of one or two pieces, be composed with as many as four. Many sisters who could not find suitable tops lengthened their scarf "hijabs" into what is now termed "khimars"---waist or knee length, and paired them with long skirts that were an overgarment (not the outfit itself but what is worn on top of). Some call these garments more polished "prayer outfits". They are what is closest to what the women of Ansaar were wearing before the ayah revealing khimar was revealed, closer even than an overhead milhafaah abaya:D. Other sisters composed their jilbabs out of one peice of cloth, wrapping it around their bodies, some women with Iranian chadors, other bright traditional African patterns, and come with saris. Anyways, I have seen sisters recently wearing maxi dresses and jumpers with their khimars covering their chests and most of their arms with a modest loose fitting t-shirt underneath, and I do think that this is jilbab so long as the maxi is an overgarment and khimar is covering most of the arms. Also, layering tunic (over top of a tee or a tank) and a skirt (over some kind of bottom) so long as the fabric is not clingy or see through, also constitutes jilbab, and maxi worn as an overgarment with a box coat (not a spandex shrug or carina top) is also Westernized jilbab. The key is, the garment has to remain an overgarment. Even a loose robe abaya worn with nothing underneath of it but bra and panties will leave the indent of your belly button exposed if the wind hits you right. Since finding garments manufactured specifically for the purpose of jilbab was expensive and difficult, many turned to ethnic clothing that was loose and non-see-through enough to do the trick, and wore traditional African and Arab inner clothing as the outer layer. Modestly decorated takchita (some ARE worn to display social status), jalibiyia (some are worn too elaborately decorated to count as jilbab to beeeeeeeee careful), djellaba, long dresses, and caftans, all work as an outer layer provided they are worn as such with the same rules about what a jilbab must be in mind. They often make the most beautiful special occasions dressings.
InshaAllah I hope this post is useful to some of you.