It has been brought to my attention that originally, both Ashkenazim and Sephardim used a square knot (also called the "double-daled knot") on the tefillin shel rosh, but when the daled knot was invented, tefillin makers began tying it in place of the square knot.
It seems that in general, the Ashkenazi custom is to use a square knot, while the Sephardi custom is to use a daled knot. There are, of course, exceptions to this: various Chasidic groups use the daled knot, and certain Sephardi communities use the square knot (such as the London community).
Some have told me that the use of the square knot is obsolete, and that one should use the daled knot, even if his family custom is to use a square. Some have even cited Rav Soloveitchik in this.
A few questions:
What are the origins of both knots?
Is there a reason why Ashkenazim use a square knot while Sephardim use a daled? Also, considering that Spanish-Portuguese customs usually reflect an older Sephardi tradition, does this mean that the older custom is to use a square knot?
Why would a daled knot be preferred, when it seems the square knot is older?
If you look at companies which sell tefillin online (whether this is a good idea or not aside), the default option is to have a daled. Square knots are usually tied upon special request. Could it be that the daled knot is much easier to tie?