Yes Cease, when you add up the length, width, and height of the box, the total of those three measurements has to be 36" or less. So a box that is 12x12x12 is right at the limit. it could be 16x10x10, or 18x10x8, or just about anything that equals 36" or less.
Just a friendly warning, be careful with fractions though! The box may be stamped 12x12x12 on the bottom, but actually measure 12.5x12.25x12.25 which would actually equal 39" according to the clerk since they can and will round up any of the three measurements. I have found it is better to err on the conservative side with both the weight and measurements so as not to give the clerk anything to ding you on.
"We're just two lost souls swimming in a fish bowl -- year after year..."