Believe nothing, no matter where you read it, or who said it - even if
I have said it - unless it agrees with your own reason and your own 
common sense. 
-- Buddha
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It definately isn't base 27, or you would need to use **27 not **26.

It is base-26 in the same way as feet are in inches base 12, or hours are in minutes base 60. The only unusual thing, in computer terms, is that it is a 1-based number system not a 0-based, but in the real world, most things are 1-based.

We measure the months 1 - 12 not 0 - 11, the metres in a kilometer, millimetres in a metre, grams in a kilo etc etc. as 1 - 1000 not 0 - 999

Update:

To reenforce this point. If the system where base-27, then the cycle would repeat every 27 symbol. It doesn't it repeats every 26th. vis. It MUST be base-26.

If it makes people more comfortable to think of it as A representing 0 and Z representing 25, so be it, but for it to be base 27, you would require 27 symbols, but there only 26.

---

Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"When I'm working on a problem, I never think about beauty. I think only how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong." -Richard Buckminster Fuller

I know it's been years since the last post on this but I came across it in dealing with the same situation. I'm not seeing how "base27 with no defined symbol for zero" really describes this numbering system either. For example, the 27th integer (if that term really applies here) in the "MS Excel" system would be AA. "AA" in a base27 system would be the 28th integer. Maybe we should call it a Mayan base26 since their base6 system didn't have a zero either (because they never discovered it).

I think you will find it's called Bijective base 26 (non 0 Alpha) to base 10