In response to the question that many have asked me about the proper length of dresses:

My opinion is known that the proper length that applies equally to all Jewish daughters and in all places is that the [length of the garment be such that the] knees be covered even while sitting.

As already stated, that which I mentioned above is the minimum amount that applies equally to all.

However, since there are places where this minimum amount is insufficient - for with regard to matters of tznius and the like, in addition to the fact that there are observances of tznius in different places that are not to be changed, there are also details that depend on the custom of the place (i.e., they depend on the custom to be stringent, but not to be lenient) - it is incumbent on the local Rav who provides Halachic decisions, to clarify and rule with regard to that particular place.

Additionally - and this too is of primary importance - the necessity to be stringent in accordance with the conditions of the location, does not necessarily mean that this is a mere stringency. For it is also possible that the conditions of the place cause it to be a Torah prohibition. And we find this to be so with regard to many matters.

Excerpt from a letter of the Rebbe, 13 Cheshvan, 5732