from wikipedia
Balklines did not end the use of the rail nurse but they did restrict its use. Soon a new type of nurse was developed which exploited a loophole in balkline rules: so long as both object balls were on either side of a balkline, there was no restriction on counts, as each ball lay in a separate balk space, a technique called the
anchor nurse.
[1]
On 1894, Chicago billiard hall owner J. E. Parker, after Schaefer and
Frank C. Ives both posted extensive runs at his hall using the anchor nurse,
suggested adding a rectangular marking straddling the spot where the balkline meets each rail, known as the anchor space and nicknamed the "Parker's box". Enclosing a space 3.5 inches (8.9 cm) out from the rail and 7 inches (18 cm) across, the box marks a region where both balls are considered in balk, even if the object balls physically fall on either side of a balkline. When first instituted, ten shots were allowed while the balls were inside the anchor space. This was reduced to five in 1896.
[1]
True to form, the next skill development response was the
chuck nurse, known as a
rocking cannon in the
United Kingdom. With one ball frozen to the cushion in the anchor space, but the second object ball away from the rail just outside the borders of the anchor space, the cue ball is gently rebounded off the
frozen ball not moving it, but with just enough speed to meet the other object ball which rocks in place, but does not change position. In 1912,
William A. Spinks ran 1,010 continuous points using the chuck nurse and broke off his run without ever missing.
[1]
There were a number of proposals to curtail the chuck nurse's effectiveness, including removing the four balk spaces on the end rails but leaving balk spaces in place on the long rails,[16] but the solution ultimately reached, and the change that brought the general rules of balkline into configuration with what is played today, was simply a doubling of the anchor space to 7 by 14 inches (18 cm × 36 cm), placing the chuck nurse out of reach. The new restriction was instituted for a 1914 tournamen
