From "The Land Where The Blues Began" the sequence from deep water to shallow, mark four being the first count on a 33' line with a lead filled pipe on its end. At 4' it's marked by a piece of white flannel, at 6' by a piece of leather, at 9' a piece of cotton, at 12' by a piece of leather split into 2 thongs, this was mark twain (two) or 2 fathoms, mark three (3 fathoms) with 3 thongs, mark four was a single strip of leather with a round hole in it, next call would be No Bottom. Calls inbetween would be quater less twain, or half twain. An example of the calls would be, from deep to shallow, as follows;

No bottom
Mark fo'
Quater less fo'
Quater less three
Half twain
Quater twain
Quater less twain
MARK TWAIN

Barry