All these threads about double and triples, have made me wonder about something:

What's the LOWEST attainable gear you can have with a Compact Double ?
(say, is it a 34x28 or 32?)

Right now, I have an Ultegra triple which bails me out on the steepest climbs after long summer rides at a hard pace, where most of the time find myself in my 30x21.

Corsaire

I run a 50/34 with a 12-27 cassette which replaced a 52/42/30 triple with a 12-25 cassette. When you compare the 34-27 combo with the 30-25 combo you can see that it is pretty close but not quite as low. If you look at Sheldon Brown's gear calculator and compare the two set ups it looks like this:

30-21 = 37.5 gear inches 34-24 = 37.2 gear inches
30-23 = 34.3 gear inches 34-27 = 33.1 gear inches
30-25 = 31.5 gear inches

These numbers are generated with a 700x23 wheel/tire and a 172.5 crank for both combos. As you can see the 30-21 is nearly identical to 34-24 and the 34-27 is in between the 30-23 and 30-25. I have about 400 miles on my compact setup now and have found it to be very nice and the low gears are plenty low even to get over a big steep climb at the end of a long ride. Persoanlly I like the compact double better than the triple.

34-27 which is a pretty darn low gear. That's something like a 39-30/31.

If you have Campy you would have a 34-29.

Try a 33 small ring on a 110 crank,then go get a freewheel with a 38 cog.

The lowest double I have seen is a 28/38 using a Stronglight Escapade crankset. You can't get that low using a typical compact double.

I used a 34 / 29 up Mt. Washington once and that was fine. Anything less and I would feel like I was moving faster in circles than forward.

34/34 with ATB cog set in back is about as low as you can get without getting some odd front ring or odd free wheel, but you'll need an ATB rear derl. That is the most practical way to get a lower gear for the compact. What is kinda cool about that set up is in regards to the 50t up front with the second cog down from that cog set. That gear is a cross chr. with a standard road cog set like the 12-26 but can be used with the 50 big ring and is nice for short rolling hills with a good grade at the top.

I have this setup on one of my bikes and could not agree more. Rarely do I need the 29. For most of my longer climbs 0.5-1m 6%grad (long where I live), the 34/23 works perfectly well.