The standard method of tying tzitzit today is to take four strings, placing them through the hole at the bottom of each corner of the four-cornered garment so that they are eight strands, with one strand longer than the rest. Five knots are made, with the longer strand twisted around the rest between each of the knots.
Rabba argues that one of the knots is a Torah obligation – the kesher elyon, or “uppermost knot.” His proof for this is that the Torah offers specific dispensation that allows woolen tzitzit with tekehlet (see yesterday’s daf) to be placed on a linen garment, the prohibition of sha’atnez (which prohibits the mixing of wool and linen) notwithstanding. Were there no requirement to actually knot the tzitziyot onto the garment, then this could be done by simply inserting the woolen tzitziyot which by itself would not be prohibited according to the laws of sha’atnez.
Rashi on our page suggests that the kesher elyon actually refers to the knot at the bottom of the series of knots and twists, which is the “top knot” in comparison to the strings that descend from it. He views this knot as significant because those strings would not last if they were not tied together. Tosafot, however, quote Rashi as holding the exact opposite position – that the kesher elyon is, in fact, the knot that is tied closest to the garment. This opinion does appear in Rashi in Massekhet Sanhedrin (88b). According to this view, after the four strings are folded through the hole in the garment and appear as eight strings, it is when they are tied together – with a single, double or triple knot – that they actually become tzitzit.
Many rishonim disagree with the traditional view that the knots are made using all eight strings; they rule that the knot is made only with the longer string that is twisted around the others. According to this approach, the kesher elyon is the first time that the longer string is tucked under the twists, creating a knot.