(updated 9/2014)
Top Plate (inside view)...

... According to to Author and researcher Isaak Vigdorchik in his book 'The Acoustical Systems of Violins of Stradivarius and Other Cremona Makers', this is the method faithfully used by both Stradivarius and Guarneri del Gesu.
... If you gently tap down the center of the marked diagonal strips, the tone will be as marked; and if you were to cut the plate apart along the diagonal lines, each strip would still give the same tap tone as when part of the whole.
According to author Vigdorchik, Stradivarius made this central strip wider so that the sound post actually rests on the strip; also there is just one octave between the highest and lowest tone along the back, going from A#3 up one octave to A#4.
. ..Notice how this contrasts to the top plate tuning: there the central tone strip is D4 and the extreme tones are G#3 and G#4 so that at each point the top is just one whole tone (two semi-tones) below the back plate.
... The actual process of tuning these plates is well described in Mr. Vigdorchik's fine book .