2.1 The 90° Tangent + 30° Rule

Beyond "did it go in?"

Beginners care about: did the ball drop? Intermediate players care about: where did the cue ball end up?

That single question — where the cue ball goes — is the dividing line between bar-room player and league player. It's also the foundation of position play (Lesson 2.2 onward).

Two laws cover ~95% of cases. They're simple but most players get them subtly wrong. Cite: Dr. Dave Alciatore, "Cue Ball Direction for All Types of Shots."

⏱ ~14 min · 4 drills · the 30° rule alone is worth the lesson

Case 1 · Straight shot → cue ball stops

When the cue ball hits the object ball head-on (cut angle = 0°) with no spin, all forward momentum transfers to the OB.

The cue ball stops dead at the impact position. This is the stun shot.

Powerful side effect: where the cue ball stops = where the object ball was. You can use a straight-in pot to deliver the cue ball to a precise spot.

• Straight shot (0° cut) + center hit → cue ball stops at impact
• Need the cue ball at point X? Find an OB at X and pot it straight
• Requires no top/bottom spin (center hit only)

Case 2 · Cuts → the 90° tangent line

Any cut angle (not straight). The cue ball is partially deflected. With no spin, it leaves at exactly:

90° to the object ball's outgoing direction.

This 90° line is called the tangent line. To predict where the cue ball goes:

• Draw the object ball's path to the pocket (yellow)
• Through the impact point of the cue ball, draw the perpendicular to that path (blue)

The blue line is your cue ball's exit direction.

• CB and OB leave impact at perpendicular angles
• Tangent line passes through the cue ball impact position, not the OB
• 100% accurate for stun (no top/bottom spin)

Case 3 · How cut angle changes CB direction

Different cuts → cue ball deflects different amounts (always along the 90° tangent line, but the line rotates with the OB direction):

• Thin cut (70°): cue ball continues nearly forward — barely turns
• Medium cut (30°): cue ball deflects sharply, ~60° from original direction
• Straight (0°): cue ball stops

So choosing the cut angle = choosing where the cue ball goes. Different cuts on the same OB send the CB to different zones.

• Thin cut → CB barely deflects (continues forward)
• Medium cut → maximum deflection
• Straight → CB stops

Case 4 · The 30° Rule (this is the workhorse)

Everything above assumes pure stun — the cue ball has no roll at impact. That's rare in real play. With normal speed and length, the cue ball is rolling forward by the time it reaches the OB.

For a rolling cue ball hitting cuts in the 1/4 to 3/4 ball range (cuts 14°-49°), the post-impact direction is:

~30° off the original direction — not 90°.

Why? At the instant of impact the CB does follow the tangent (90°). But the forward roll re-engages within milliseconds and pulls the cue ball back toward forward, settling at ~30° deflection.

This is Dr. Dave's "30° Rule" — the actual workhorse of position planning. Use his "peace-sign" hand visualization: spread your index and middle fingers ~30° and overlay them at the impact point.

• Rolling CB + 1/4-to-3/4-ball cut → ~30° deflection (not 90°)
• 90° rule = stun only; 30° rule = natural roll
• ~80% of position planning uses the 30° rule

Quick reference

Pre-shot routine:

1. Identify the OB → pocket line (yellow aim line is shown)
2. Decide: stun or rolling? → choose 90° or 30°
3. Mentally draw the corresponding deflection line at the impact point
4. That's where the cue ball stops — is it safe? Useful for the next ball?

Four drills below — predict, then control, the cue ball's final position.