I have a large (10) aresanl of used balls, but play on very different conditions. Normally I take 3 strike balls with me, plus a polyester spare.

When I do training, I rotate my arsenal very often, just to check release variants with different balls and have my arsenal checked on different lane conditions (which vary strongly - on a fresh house I need a deep oil ball with eraly reaction, but when lanes break down in the eveneing sessions, it can be that I can only play light oil balls. And then, every pair of lanes can be very diffrent!).

When it gets to serious playing I have my favorite arsenal for different houses, balls of which I either know they normally work very well in this house on a "normal" condition, and then 2 backups for either heavy, medium or light condition - balls of which I know I can adapt without problem to differnt lane conditions in the first place and over time, which react consistently.

But this "inner circle" of balls is currently 5 out of 10, the rest is waiting to be confirmed through training and experiments, but not a tournament choice.
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DizzyFugu --- Reporting from Germany

"All that we see or seem, is but a dream within a dream..." - Edgar Allen Poe

I take 3 to league, always my spare ball, but almost never are the other two the same, week to week.  I normally rotate one out every week; on occasion I'll switch both.  For example, last week I had my Silver Streak Particle and my Bullet.  Tonight I'll take the SSP and my Silver Streak Particle Pearl (just want to see how far apart they are on the condition.)
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"A little sincerity is a dangerous thing, and a great deal of it is absolutely fatal." Oscar Wilde
Ragnar sure likes to throw his purdy Uranium Buzzsaw.

I always take the Crisis, and I almost always take the Thing Returns, because I simply don't have anything else that covers what they cover.

It's mostly the medium-heavy oil balls that get rotated.  One week I'll bring the Deuce, the next week it may be the Pro Purple or the Carbide Bomb, and the week after that it'll be the old Super Power.  Just to see if I get a "look" from something that works well on whatever we're bowling on at the moment.

It's mostly because I have lots and lots of balls, and very little talent.

Still looking for that magic ball, I guess... (sigh)
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seadrive
Cogito ergo bowl

I typically bring the same group of balls with me to league week to week, unless the shot drastically changes or I know it's a vacation week for the kids.  When I was in multiple leagues or different centers I would change things up.

The time to bring out unknowns for me is typically practice sessions.
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-Chris: DJ's Pro Shop : Auburn, MA

Certain balls work on certain conditions, I use this idea when I bowl in league, usually bring between 3 and 5.  On sunday I bring a little bit of everything, I don't know what to expect. Monday I bring med-light stuff.  Thursday I bring a big hooking ball and some light oil stuff.  Friday I bring things with backend and my urethane.  Rotation is worth it if you bowl in many leagues, where the shot may not always be the same.  
Or sometimes I go to league just to practice a couple of things and bring whatever I want!!

I have a "read" ball(Vendetta Solid) that is always in the bag.I can line up quickly and get a solid read and adjust from this ball with ease.The other two will vary from time to time but will always be a pearl and an oiler of some kind.I rent a locker at the two houses I bowl at and keep a dry ball and a super hooker there for those times the shot is extreme.

For tournaments I take a 5 ball arsenal and a spare ball as I do not have the benefit of the locker.This line up varies but will usually consist of balls from the same company.
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President and CEO Visionite Stormy Buzzwick LTD