Gray's Loop
A version of the Constriction knot for attaching a semi-permanent loop of nylon monofilament to the end of a traditional PVC coated, terylene core flyline to facilitate a loop to loop connection with the leader. Such a loop might last a whole season.

Starting with a length of about 18 inches [half a metre] of nylon makes it easier to pull the knot tight. Also start with as small a loop as possible, held between the right thumb and forefinger [figure 1], if you want to create a small loop [although a loop of anything up to around three inches long works fine]. Use a minimum of three turns. Carefully draw the knot together while sliding the whole knot towards the end of the fly line. With the turns of the knot touching, pull steadily on both ends until the knot grips the fly line tightly. Check the security of the knot by pulling steadily with a finger in the loop. When satisfied, trim the ends closely. The strength of the nylon will vary with the weight of fly line and breaking strain of leader. As a guide use the following :
FLY LINE #
LEADER
LOOP
5
3-5 lbs
10 lbs
6
4-6 lbs
12 lbs
7
6-8 lbs
12 lbs
8
8-12 lbs
15 lbs
9
10-12 lbs
15 lbs
10
12-15 lbs
20 lbs
This knot is not suitable for fly lines with a kevlar or monofilament core.
If it was considered important that the loop should emerge from the centre of the fly line, it could be attached using a needle, as in a needle knot.

Courtesy of GRAYS OF KILSYTH - www.graysofkilsyth.com