Cut Shot in Pool and Billiards

... various effects, measures, and techniques related to a pool cut shot.

Dr. Dave's answers to frequently-asked questions (FAQs),
mostly from the BD CCB and AZB discussion forums

maintained for the book: The Illustrated Principles of Pool and Billiards,
the DVD series: The Video Encyclopedia of Pool Shots (VEPS) and
The Video Encyclopedia of Pool Practice (VEPP), the Billiard University (BU),
and the monthly Billiards Digest "Illustrated Principles" instructional articles


For more information, see Chapter 3 in The Illustrated Principles of Pool and Billiards
and Disc I of the Video Encyclopedia of Pool Shots


back cut

What is a back cut and why is it tougher than a regular cut?

A back cut is a cut shot where the cue ball is shot well away from the target pocket (e.g., when the cue ball is much closer to the target pocket rail than the object ball). These shots can be more difficult than cut shots looking toward a pocket because the required cut angle can appear larger than it actually is, because you are looking away from the pocket. As a result, most people tend to overcut back cuts (i.e., they are hit too thin). For more info, see shot #6 in VEPS-I. Also, with back cuts, because you are looking away from the pocket and the near rail adjacent to the pocket, the visual cues for the pocket location are not as clear as with non-back-cuts (where you are looking toward both rails adjacent to the pocket, helping to provide better visualization of the pocket location).

A back cut up to (and even more than) 90 degrees is possible with the help of outside english (OE) . Diagram 2 in "Throw - Part VI: inside/outside english" (BD, January, 2007) shows how much OE is necessary for "gearing" OE (resulting in no throw). For a 90-degree cut, 80% english is "gearing," so to throw the ball in the SIT direction with a very thin cut, you need more than 80% english. For more info, see:

HSV B.33 - Outside english gearing, and cut and spin-induced throw
NV B.86 - Cut-induced throw (CIT) and spin-induced throw (SIT), from VEPS IV
HSV A.142 - Vernon Elliott cross-side bank with chalk on the object ball to increase throw and spin transfer

from Patrick Johnson:

When we shoot back cuts we're looking away from some important visual cues, namely the pocket and the rails that point to it. We learned to use these cues to help us see the OB-to-pocket line and find the contact point/cut angle without all the walking around (the usual reason for innovation). But we learned them subliminally, so we don't notice that we're using them and don't know what's wrong when they're gone.

Once we do know what's wrong, the simple solution is obvious: revert to walking around and sighting the OB-to-pocket line as we did before learning to use the visual cues (or resort to the stick-pivot method or any other "mechanical" method to get a fix on the contact point/cut angle).

AIMING TIP:

The main difference between backcuts and other cuts is that there's no nearby rail behind the OB to show you the direction to the pocket. So a useful trick for aiming backcuts is to "replace" the rail by imagining one right behind the OB pointing to the pocket. Sounds a little nutty, but this often helps to see where to hit the OB. It even helps on non-backcuts where the OB is farther from the rail (this is why OBs in the middle of the table can be harder to aim).

The rail behind the OB is an important visual "cue" - I find OBs on or near the rail much easier to aim.


ball-hit fraction vs. cut angle

How is cut angle related to ball-hit fraction?

Illustrations, equations, plots, and numbers on this topic are available here:

TP A.23 - Ball-hit fraction vs. cut angle

Here's a useful illustration from “Aim, Align, Sight - Part I: Introduction and Ghost Ball Systems” (BD, June, 2011) defining the standard ball-hit fractions:

ball-hit fraction illustrations

from Bob_Jewett (from AZB post):

Here is a plot of the cut angle you get versus the fullness of hit. For example, a fullness of 0.5 -- an exact half-ball hit -- gives a cut angle of 30 degrees. (This ignores throw which can be several degrees depending on speed, spin, cut angle and ball conditions, but that is mostly a separate issue.) A couple of other fullnesses to note are 3/4 full which gives an angle slightly under 15 degrees and 1/4 full which produces about a 48-degree cut.

cut-angle ball-hit-fraction plot


estimating cut angle

How can you estimate the cut angle for a shot?

On Disc I of the Video Encyclopedia of Pool Shots, we show how to use your hand to recognize 1/4-ball, 1/2-ball, and 3/4-ball hits. We also show how to calibrate your hand so you can judge these shots (and the corresponding cut angles) fairly accurately. Knowing these three angles well can help you estimate the angle required for any shot. The cut angles for various ball-hit fractions can be found at the end of TP A.23.

A simple way to visualize and estimate cut angles is to imagine an analog clock face (or use one on your wrist). If noon (12) is straight (0 degrees), 11 and 1 are at 30 degrees (1/2-ball hit), 10 and 2 are at 60 degrees (about an 1/8-ball hit), and each minute is 6 degrees. For example, halfway between 12 and 1 ( at 2 1/2 minutes) is 15 degrees (about a 3/4-ball hit), and halfway between 1 and 2 is 45 degrees (about a 1/4-ball hit).

Below is a template posted by oldschool1478 (in AZB post) that can be useful to estimate cut angles and required aim points when practicing. Here's a printable PDF version. If the center of the cutout is placed over the necessary ghost-ball position with the arrow pointed back to the cue ball, sighting to the pocket gives the necessary cut angle, clock position, ghost-ball overlap, and aim point (center of small circle) relative to the object ball. For example, the required aim for a 30 degree cut (assuming no throw) is through the edge of the object ball. The dashed ghost-balls in the small illustrations are useful to get a sense for the amount of ball overlap required on a particular shot.

cut-angle template

from Bob Jewett:

The tip of the cue is at the center of the ghost ball. The distance X is the base of an isosceles triangle. If X is measured in quarter inches, it gives the angle, pretty nearly.

cut angle estimation

Here are x (in) x (in/4) actual angle and error:

1 4 3.82 0.18
2 8 7.65 0.35
3 12 11.48 0.52
4 16 15.32 0.68
5 20 19.19 0.81
6 24 23.07 0.93
7 28 26.99 1.01
8 32 30.93 1.07
9 36 34.92 1.08
10 40 38.94 1.06
11 44 43.02 0.98
12 48 47.16 0.84
13 52 51.36 0.64
14 56 55.64 0.36
15 60 60.00 0.00
16 64 64.46 -0.46
17 68 69.04 -1.04
18 72 73.74 -1.74
19 76 78.59 -2.59

 

from Dead Crab:

I have a way of estimating cut angles to within a degree. It works for me. Estimating to within 5 degrees would be a cinch.

1. Place a small piece of tape (blue painter's tape is good) on your cue shaft 15" from the tip

2. Place tip of cue on the CB-OB line so that the 15" mark on the cue is over the top of the CB (obviously, not touching it).

3. Leaving the tip stationary, swing the butt of the cue so that the alignment of the cue long axis is parallel to the line of the OB-intended pocket.

4. Estimate the distance (in inches) from the tape mark on the cue to the CB-OB line (i.e. drop a perpendicular to the CB-OB line from the mark on the cue. Multiply this distance by 4. That is your cut angle, in degrees.

With a little practice, the difference between 3.5" and 4" is obvious, and clearly distinguishes a 14 degree from a 16 degree cut angle. For longer shots, you can use the cue joint rather than the 15" mark, and multiply by 2 instead of 4 to get the cut angle.

From a practical viewpoint, it is important to note that the tip of the cue need not be at the GB center. Since the GB center may be 7' away, a more practical approach is to place the 15" mark on your cue over the CB, with the tip on the estimated CB-GB line (as discussed, for long shots the CB-OB line is often adequate for longer shots). Then pivot the cue around the tip until it lies parallel to the OB-pocket line. Estimate the base of the triangle in inches, multiply by 4, and you are done. For really close shots the method can be used by extending the estimated CB-OB line "behind" the CB. This saves a lot of reaching. For intermediate and long shots, the 30" point of the cue is often easiest to use (just 1" above joint), and then the multiplier is 2 instead of 4.

If you know the cut angle, you know where to aim from the CB center. 1mm per degree up to 30 degrees and above that, 0.8mm/degree off ball edge over 30 degrees covers it pretty well. Ball landmarks come in handy. A visible gap becomes apparent between cloth and ball at about 10mm off center (napped), 7-8 mm worsted. A 20 degree cut can be aimed at 7:30 or 4:30 on the clockface, a 25 degree shot at 8:00 or 4:00. A 23 degree cut is 2mm shy of a 25 and 3mm more than a 20.


"impossible" cut shots

Is it possible to make a shot with an effective cut angle greater than 90 degrees?

Yes. Here are several examples:

See also:

HSV A.142 - Vernon Elliott cross-side bank with chalk on the object ball to increase throw and spin transfer

Impossible cuts can also be made by jumping or hopping the CB so it hits the OB while airborne. More info and video demonstrations of this can be found here:

jump shot over cut effects


margin for error

How does the difficulty of a shot vary with cut angle?

The following analysis (which includes several useful graphs) shows how the margin for error varies with both shot distance and cut angle:

TP 3.4 - Margin of error based on distance and cut angle

Also, the effective size of the pocket varies with angle and speed. For more info, see: pocket "size" and "center"

from Bob_Jewett:

An article that discusses how the angle increases the difficulty of a shot (which is to say, decreases the allowed error), is at: http://www.sfbilliards.com/articles/1994.pdf (April)
The article includes a simple graphical way to see how much the cut angle increases the difficulty over a straight-in shot of the same length.


overcutting

Why do people overcut cut shots (i.e., hit OB ball too thin)?

Sometimes the aim is just off. When using english (especially outside english), swerve and/or throw might be the culprit. For more info, see:

aim compensation for squirt, swerve, and throw

On firm follow shots and/or jump shots, sometimes the OB is overcut because the CB is airborne when it hits the OB. For more info, see:

jump shot overcut effect
follow shot ball-hop effects

from Bob Jewett:

There is some throw on all shots. That is, the surface of the cue ball is somewhat sticky, and as it passes across the surface of the object ball, it will pull the object ball off the ideal path, which is the line through the centers of the two balls at the instant of contact.

If the cue ball is moving faster, there is not as much friction. This is contrary to the simple explanation of friction which says that the coefficient of friction does not change with the force of the contact. The reduction seems to be over a factor of two as you crank the speed up

If you have stun on the cue ball (no follow or draw), the throw you see on a cut shot is maximized for that speed because all the rubbing is to the side. As you add draw or follow, the effective friction is reduced because part of the rubbing is up or down, and that doesn't contribute to the throw.

Most players have no idea that this stuff is going on, and to some extent it is better for them not to know. They learn subconsciously to make corrections for it, and if they start thinking about it during shots, their brain will get in the way of their arm.

There are fairly simple demonstrations for this. It is discussed on Dr. Dave's web site and in Bob Jewett's May '06 BD article.

The cue ball also jumps on hard shots as mentioned above, but this causes thinner cuts mostly for short distances to the object ball or excess stick elevation.


speed and travel distances for the CB and OB

How do CB and OB speeds and travel distances vary with cut angle?

See "Fundamentals - Part VII: speed control" (BD, March, 2009). It includes distance graphs for both stun and rolling CB shots.

For more information, see the ball speed and travel distance resource page.


undercutting

Why do some people undercut some cut shots (i.e., hit OB ball too full)?

Some people have trouble visualizing the required ghost-ball center for a cut shot, and get influenced too much by the ball contact point. For more info and illustrations, see "Fundamentals - Part II: aiming" (BD, October, 2008).

Sometimes, cut-induced throw (CIT) is the cause; and when english (especially inside english), swerve and/or throw might be the culprit. For more info, see:

aim compensation for squirt, swerve, and throw

One way to compensate for undercutting the ball is to use outside english, but this isn't the best approach for all people. For more info, see:

outside engligh resource page


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