Post 03 controlled the white on a straight pot. Most pots are cuts, and on a cut the white goes off to the side. This lesson shows where: the square line, the 30° rule, follow and screw on a cut, how power and the cut angle change the path, how much speed the white keeps, and two checks before every shot: in-offs and kisses.

Series map. ★ Basics: 01 Cue action and elevation → 02 Aiming and potting → 03 Tip and power → 04 Where the white goes (this post) · ★★ Club: 05 Side spin → 06 Cut and side into pockets → 07 Cushions → 08 Doubles · ★★★ Advanced: 09 Escaping snookers → 10 Multi-ball shots → 11 Safety → 12 Position play · ★★★★ Hard: 13 Break-building → 14 Special shots · Reference: 15 Rules, tactics and practice · ★★★★★ Pro: 16 Cue-ball control.

Learning Outcome

  • Find the square line on any cut, and send the white along it with a stun.
  • Send the white a little forward (stun run-through) or about 30° forward (rolling).
  • Use power and the cut angle to predict how follow and screw bend the white.
  • Know how much speed the white keeps on full, half and thin cuts.
  • Check the white's path for a pocket (in-off) and for other balls (kisses).

Chapter 1 — The square line

1 · Three tips, three paths

Top view of a half-ball cut into the corner. The red goes up the yellow dashed line to the pocket. From the contact point three white tracks leave: yellow (top) bends forwards, white (stun) goes straight along the square line, blue (screw) goes back. A small white face at the end of each track shows the tip spot

The same pot, the same aim, three tips:

Tip The white goes…
▲ above centre (follow) forwards, bending away from the square line
■ a little below (stun) along the square line, at a right angle to the red's path
▼ low (screw) back, behind the square line

The square line (the small white square) is your anchor. Find it first, then decide: forwards of it, on it, or behind it.

2 · Stun run-through on a cut

Top view of the same half-ball cut. Three white tracks leave the contact point: white, straight along the square line, labelled stun 0 degrees; yellow, a little forward, labelled run-through 15 degrees; orange, further forward, labelled rolling 26 degrees

Between stun and follow there is a whole range. In the model, half ball, power 3, white 40 cm away:

Tip The white ends…
0.19 R below centre (stun) on the square line
0.04 R below centre (stun run-through) 15° forward of it
0.35 R above centre (rolling) 26° forward of it

Trick: need the white just a little forward of the square line, for a red a few inches from it? Stun run-through: hit just below centre, a little higher than your stun spot.

3 · Rolling white: about 30°

Three top views: three-quarter ball, half ball and quarter ball cuts with a rolling white. In each, the white's track bends and then runs straight. A yellow wedge between the white's old line and its new line is about the same size in all three

When the white is rolling (hit above centre, or slow from far away), it ends up turned about 30°, a third of a right angle, from the line you sent it on. That holds for most cuts from ¾ ball to ¼ ball. In the model: ¾ ball 28°, ½ ball 34°, ¼ ball 27°.

Trick: lay your cue on the table along the white's line, then turn it a third of the way to a right angle. That is where a rolling white goes.

Chapter 2 — Follow and screw on a cut

4 · Watch it: follow on a cut

Clip: half-ball cut into the corner, tip above centre, power 3. The square line is shown dashed. The white first slides along the square line, then bends forwards

The white starts along the square line, then the topspin grips and bends it forwards.

5 · Watch it: screw on a cut

Clip: the same cut, tip low, power 3. The white leaves along the square line, then the backspin pulls it back behind the line

Low tip: the white leaves along the square line too, then the backspin pulls it back behind the line.

6 · Power decides where the bend starts

Two top views of the same half-ball cut. Left, follow: at power 2 the white bends forwards almost at once; at power 4 it first runs a long way along the square line, then curves. Right, screw: at power 2 it bends back almost at once over a short distance; at power 4 it runs along the square line first and swings back in a wide curve

Every white leaves along the square line. The spin then takes time to grip. More pace = further along the square line before the bend (model, distance until the white has turned 15°):

Power 2 Power 4
follow 4 cm 39 cm
screw 1 cm 36 cm

So the same tip spot gives very different paths. A soft shot turns at once; a firm shot keeps the square line for a while, then swings in a wide curve.

Watch out: a firm screw on a cut does not come straight back. It runs along the square line first. Check that part of the path too.

7 · The cut decides how much screw comes back

Three top views with the same low tip and power 3: three-quarter, half and quarter ball cuts. On the three-quarter ball the white comes back well behind the square line, 62 degrees; on half ball 40 degrees; on quarter ball only 23 degrees

The same screw (tip 0.45 R below centre, power 3) on three cuts:

Cut The white ends behind the square line by
¾ ball 62°
½ ball 40°
¼ ball 23°

The fuller the hit, the more the screw shows. On thin cuts the white keeps most of its forward speed (section 9), so screw only nudges it.

8 · Nearly full cuts

Top view of a seven-eighths ball cut. The yellow follow track runs on almost straight past the red, 17 degrees off the cue line; the blue screw track comes back almost straight towards the start, 26 degrees off

On a nearly straight pot (here ⅞ ball, a 7° cut), the white goes almost straight on with follow or almost straight back with screw. In the model, follow ended 17° off the cue line and screw 26° off it.

This is why nearly straight pots are hard for position: the white can only go forward or back along one line. Leave yourself a bit of angle when you can (post 12).

9 · How much speed the white keeps

Three top views with a stun at power 3: three-quarter, half and one-eighth ball cuts. Labels: the white keeps 25 percent of its speed on three-quarter ball, 50 percent on half ball, 87 percent on one-eighth ball

On a stun, the speed is split between the two balls:

Cut The white keeps The red gets
¾ ball 25% 97%
½ ball 50% 87%
⅛ ball 87% 48%

(The two shares are the sine and cosine of the cut angle; they add up in speed squared, not in plain numbers.)

So on a thin cut, the white travels a long way and the red only a little: play thin cuts firmer to reach the pocket, and expect the white to run on. On a full hit, the white goes only a short way.

Chapter 3 — Check the white's path

10 · Avoid the in-off

Three top views of the same half-ball pot into the top-right corner. Stun: the white runs along the square line straight into the bottom-right corner pocket, cross. Follow: the white bends forwards, hits a cushion and stays on the table, tick. Screw: the white comes back and stays out, tick

Here the square line points straight at a pocket. A stun sends the white in after the red: an in-off, a foul ✗. Follow or screw move the white off the line ✓.

Clip: the half-ball pot into the top-right corner with a stun. The square line is drawn dashed from the contact point to the bottom-right corner pocket. The red goes in, and the white runs along the square line into the bottom-right corner

Trick: before every cut, look along the square line. If it hits a pocket, choose follow or screw, or change the pace so the white stops short.

11 · Check for kisses

Two top views of the half-ball pot with a blue ball half a metre along the square line. Stun: the white runs along the square line and hits the blue, cross. Screw: the white bends back and misses the blue, tick

A ball on the white's path changes everything: the white stops, the other ball moves, and your next shot is gone. Here a blue sits 52 cm along the square line. The stun hits it ✗; screw takes the white behind it ✓.

The check is the same as for in-offs: picture the path for your tip and pace, and look along all of it.

Try it at the table

Drill Set-up Look for
Square line a half-ball pot; lay a cue on the table along the square line stun: the white runs along the cue
Four paths the same pot with screw, stun, stun run-through, follow four different landing spots; mark each with a coin
30° check top spin on ¾, ½ and ¼ ball cuts the white turns about the same each time
Power and the bend follow at power 2, then at power 4 the firm one runs along the square line first
In-off spotter a pot whose square line points at a pocket play it with follow, then with screw, and stay out both times

Cheat-sheet
Stun along the square line, at a right angle to the red's path
Stun run-through just below centre: a little forward of the square line (about 15° on half ball)
Follow forwards; ends about 30° off the line you sent the white on
Screw behind the square line; more on fuller cuts (¾: 62°, ½: 40°, ¼: 23°)
Power firmer = runs along the square line longer before bending
Nearly full only straight on or straight back
Speed kept ¾: 25% · ½: 50% · ⅛: 87%
Before every shot look along the white's path: pockets, balls

What's next

05 · Side spin. What side really does: squirt, swerve, throw, and side off a cushion, and how to aim with it.

References

  • Dr. Dave Alciatore, the 90° rule and the 30° rule (articles, videos and technical proofs). https://billiards.colostate.edu/
  • WPBSA, Official Rules (2022): in-off is a foul. https://wpbsa.com/rules/
  • The speed split on a stun is exact for smooth balls (sine and cosine of the cut angle). The angles, distances and tip heights come from the physics model of the balls at snooker sizes; real tables vary a little, the patterns hold.

Note: this post is in progress. Planned additions: the same paths into a middle pocket, and where the white goes after the first cushion.