A very popular and reliable knot which can be used for tying two lines of similar strength together.
Lay the two lines side by side as in the diagram.
Grip the two ends between thumb and fore finger and twist them in opposite directions at least five or six times on each side.
Pass the two free ends through the centre twist so that the two ends come through the centre of what will be your knot.
Wet the knot (as with all knots made in mono filament) pull tight and trim the ends.