The gallery is the engine room of the pyramid where the main calculations required to calibrate the geometry are performed. It will be analysed over the coming pages, and because the design is highly complex it is worth summarising the architectural concept of the gallery that will be discovered, which is quite straight forward, before embarking on the analysis.
The roof of the gallery is not continuous straight line, but is made up of a series of ratcheted stones as can be seen by zooming in on an arbitrarily selected area of the roof. The ratchets themselves are cut at an angle that is perpendicular to the slope line of the roof stones, and are not vertical.
The reason for the roof being built in this way is that a nominal 'test' angle of the roof of the gallery is defined from a right angled triangle that conforms to the roof's architecture. If you display the triangle you can see that it is a primitive right angled triangle with a hypotenuse of 221 and a side length of 200, the units of length being irrelevant. These values have already been used for the cubit to radian ratios seen earlier (page C5); 200 in the gallery and lower northern shaft and 221 in the lower southern shaft and the value of 221 was so important that the architects painted it in red numerals behind the door at the end of the lower southern shaft.
This primitive triangle is essential for understanding the gallery and calibrating the architectural design because it defines the 'test' roof angle of the gallery as arcCosine(200/221) = 25.17976°. This angle can be calculated to as many decimal places of accuracy as we need, which is the very reason for it being defined in this way.
The triangle is also used in the gallery end walls and floor construction in two specifically sized formats where it is scaled so that the hypotenuse of the triangle is 0.221 and .0221 cubits long and the other integer length side is 0.2 and 0.02 cubits long. These triangles have the following dimensions, shown in cubits, metric and inches so that they can be compared to Petrie's original survey dimensions if desired.