What makes a double IPA a double??

I have never brewed an IPA per se I have brewed lots of beer that was hoppy enough to maybe be an IPA like if I hoped up my pale ale recipe a bit. Just wondering what makes a DOUBLE a DOUBLE? Is it the hop profile the gravity the mash or the process?? School me a bit
Cheers
JJ

"Double" is what you will be seeing after consuming a couple Designations such as "Triple" and "Quadruple" follow this same system.

Technically, it's an "Imperial IPA", not a "double".

It's a combination of higher OG (usually ~1090 or so) and more hop bitterness to balance it out. It still wants to be reasonably dry, though, you don't want it to be cloyingly sweet even with all that malt (it's not a barleywine). IBUs will usually be basically off the charts.

By the style guide, what the_bird said:

Imperial or Double India Pale Ales have intense hop bitterness, flavor and aroma. Alcohol content is very high and notably evident. They range from golden to amber in color. The style may use any variety of hops. Though the hop character is intense it’s balanced with complex alcohol flavors, moderate to high fruity esters and medium to high malt character. While hop character may be very intense, it may not be harsh in quality. Imperial or Double India Pale Ales have medium-high to full body. Diacetyl should not be perceived.
Original Gravity (ºPlato):
1.075-1.100 (19.5-23.5 ºPlato)
Apparent Extract/Final Gravity (ºPlato):
Page 13 of 35 1/13/06
1.018-1.028 (5-7 ºPlato)
Alcohol by Weight (Volume):
6.0-8.4% (7.5-10.5%)
Bitterness (IBU): 65-100
Color SRM (EBC): 5-13 (10-26 EBC)

Those FG's may not seem dry, but consider where they start.

This is a style where it is perfectly acceptable to use a modest amount of a simple sugar (like corn sugar). Not to extreme, but a pound or so, maybe ~10% of the total sugars. This will add to the OG, add to the alcohol, but will ferment out almost completely and help keep the final gravity somewhat low.

There's a great podcast on the Jamil Show about Imperial IPA dated 9/10/06.

thanks for the replies guys. I think I will start with an IPA just a bit on the smaller size for now. Any suggestions on a yeast that can be started in an IPA and a couple of batches later capable of fermenting out such a big beer as a double IPA with an OG of 1.090 ??
Cheers
JJ

You want a yeast that is a high attenuator. 1056, WLP 001 CA Ale and the like are pretty standard and should get you about 80% attenuation. Just make a big starter or pitch on a yeast cake.