Bubble or Double?

The eternal crusade of cookbookery is to find the right words for actions that you can do without thinking, much less speaking.  Frustrating stuff. 
Recently I’ve had a lot of emails asking about starters that “bubble” but do not “double.”  Many are unsure as to whether or not their starters are ready to use in this undefined state. Well for the sake of the masses, I am answering that question here as best as I can with the words stored in my semi-permeable noggin.
Ideally, when a starter bubbles up, it will double- even triple in volume.  But following the unpredictability of living things, this is not always the case.  Sometimes your starter will have bubbled up, the beneficial organisms spread throughout the starter, but the starter will have only “risen” a few inches. 

This starter is within about 8 hours
of being ready to use.  See how the
bubbles do not reach all the way to
the top?  Once they have spread through
all the starter, the starter is ready to go!

The most important factor is whether or not the bubbles have worked their way throughout the entire dough.  Usually in my starter, the bubbles work their way from the bottom up, spreading until the entire starter has bubbles.  At this point, I can open my jar, look down, and see bubbles on the top (or just under the surface) of my starter.  When this has happened, I know that my starter is ready to use.  There are yeasts and organisms in every nook and cranny of my starter, and they are ready to be put to work.
Your starter activity is a good predictor of what your dough activity will be.  If your bubbles are ginormous and working through the entire starter in less than 12 hours in the fridge, you know your dough is going to do the same.  If your starter is bubbling thoroughly but the bubbles are smaller and the rise not as impressive (or quick) you can anticipate that your dough will do the same.  Remember, you are baking with living, breathing organisms and there will be mild fluctuations.
If your bubbles are smaller and your loaves not as fluffy as you’d like, be patient.  Your starter is probably working through a little bit of an imbalance.
Make sure you are feeding your starter as soon as liquid lightly covers the top of your starter, and your starter will find its balance eventually. 
I have had email after email of people telling me that after a little patience, the opened their fridge one day to find that their starter had gone “crazy” and they now didn’t know what to do with such a happy starter.  We’ll save to the answer to that happy problem for another day.  In the mean time, keep trying, be patient with your starter and yourself.  You are learning a skill now nearly lost in western kitchens, GO YOU!
Below:  Here are some photos of a yeast experiment I did a while back.  Ignore the relatively short lapse of time between small bubbles and big bubbles (one day), that starter was in a different situation and I knew it was within a day or so of overcoming the tipping point to great bubbles.

This starter would make adequate bread, but the loaves would probably be less fluffy
and possibly have a stronger flavor.  The bubbles are small, and you can see there
is liquid on the top of the starter.  It needs to be fed and allowed to bubble again.

This starter is excessively "happy".  See how there are bubbles everywhere?
I am not sure if it had "doubled" in volume, but I can see bubbles all the way
through, and they are nice big bubbles so I can make an educated guess that
this bread would be fluffy and have good flavor.  As it turned out, they were.