FAQFAQBilliards and Pool Pocket Information

... effects and physical characteristics related to pool table pockets.

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


for more information, see Section 3.06 in The Illustrated Principles of Pool and Billiards
and Disc I of the Video Encyclopedia of Pool Shots


corner pocket rattle and how to avoid it

How do I aim to prevent the OB from rattling out of a corner pocket?

This happens when the ball hits the near rail cushion or point with speed before entering the pocket. Here's a good demonstration of this effect:

NV 3.13 - Corner pocket near rail effects

Also, super slow motion clip HSV 3.8 shows why it happens fairly clearly. The cushion imparts sidespin to the ball, changing the rebound angle off the inner wall of the pocket. The following instructional articles also have some good illustrations, descriptions, and online demos of this and other related effects:

"Just How Big are the Pockets, Anyway - Part I" (BD, November, 2004).
"Just How Big are the Pockets, Anyway - Part II" (BD, December, 2004).
"Just How Big are the Pockets, Anyway - Part III" (BD, January, 2005).


"helping" english

Can spin transfer be used to "help" an object ball into a corner pocket?

Theoretically, yes; however, practically, attempting to use spin transfer to make the pocket bigger is not very effective with typical conditions over a wide range of shots. For more information, see "Get-In English, At Last" (Bob Jewett, BD, June, 2013).

Also, here's more info on spin transfer.


from Bob Jewett:

It's easy to see how much side spin you can get on an object ball by banking a stripe and "twisting" (transferring side to) the object ball. It's not much. Maybe that little bit of side spin is useful for getting balls to drop when they are barely at the edge of hanging up, but I think any advantage is negligible, especially compared to the aiming issues when using side spin.

I think the real advantage of this technique is that it gives you confidence.


from Patrick Johnson:

The amount of spin you can transfer to an OB is too small to make a difference unless the visible pocket opening is very small. Shooting at a small target is exactly when you don't want to compromise your accuracy with sidespin, especially since it rarely is helpful.

Transferred spin wears off quickly, so it could only make a difference on short shots or on shots you hit harder. You probably don't need it on short shots because you can hit the pocket opening pretty accurately already (unless you add sidespin to the equation), and hitting shots harder makes it much more likely the OB will jaw (in addition to also reducing accuracy even more).

In other words, it's a bad idea and a myth. They're common in pool (another related one is "rail hugging english").

Accuracy is reduced not just because of squirve, but also because the amount of sidespin and the amount of throw increase together - so the more effective "helping english" might be the less accurate the shot is. It's a losing proposition every way you look at it.

... if the shot really needs "helping english" you should choose another shot.

from Jal:

Some exceptions might be:

- The OB is frozen to another ball and the carom angle will force it uncomfortably close to the far point of a pocket. The fact that they're frozen, however, allows you to take considerable liberties with the contact point, as long as you do carom it off the other ball.

- The OB is close to a pocket, but an interfering ball makes for a similar situation. Here, a relatively small error in the impact/throw direction can be more than compensated by imparted spin.


pocket "size" and "center"

Does the effective size and aiming point for a pocket vary with approach angle?

Yes. See the diagrams and examples in the following instructional artilces:

"Just How Big are the Pockets, Anyway - Part I" (BD, November, 2004).
"Just How Big are the Pockets, Anyway - Part II" (BD, December, 2004).
"Just How Big are the Pockets, Anyway - Part III" (BD, January, 2005).

For example, with a corner pocket, at shallow angles to the rail (about 2 degrees from the rail), because the object ball can glance off the rail well in front of the pocket and still go in, the effective "size" of the pocket is much larger at that angle.

As shown in Diagram 3 in "Just How Big are the Pockets, Anyway - Part I" (BD, November, 2004). The "offsets" are measured relative to the geometric center of the pocket at the leading rail edge of the pocket jaws. The offset vs. entry angle plots near the ends of TP 3.5 and 3.6 show how the "target center" varies with entry angle. For a slow shot, the point varies by as much as about 0.6 inches for a typical corner pocket and about 0.3 inches for a typical side pocket.

For experimental results showing how the pocket size and center vary with angle, speed, and spin, see:

"Where's the Pocket? ... They change size and shape," (Bob Jewett, BD, May, 2013).
"Get-In English, At Last," (Bob Jewett, BD, June, 2013).

Here's a useful app from slach on AZB that lets you select a pocket and shot speed (slow or fast) and move the CB and OB interactively (with the mouse). It then reports shot margins for errors and other interesting information for any shot:

Pool Shot Analyzer (requires Java to run)


pocket and table difficulty factor

How can you determine how difficult a table plays, and how to I use this information to adjust scores from player rating systems?

See pocket and table difficulty factor.


point compression

Is it possible to deform the point (or knuckle) of a pocket enough to force a ball in?

Yes. See:

NV B.28 - Pocket point compression shot

Cushion compression away from the pockets can also be used to help make certain shots. For example, see:

HSV B.20 - rail cushion compression shots


"tight" pockets

What makes pockets "tight" or "tough"?

See the Table Difficulty Factor (TDF) resource page.


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