This question has to do with the Shimano Road Compatibility charts. Here are the charts:

Road front drive
Road rear drive

First, how to read them (I'll only list 6000 and 7000 series parts). Second, how much to believe them.

What's the rule for reading embedded boxes? The ST-6510 shifter is (obvously) compatible with FD-6500. So if the rule is to trace compatibility through connectors of the enclosing box (the only way to get between those two), ST-6510 is compatible with the following front derailleurs: FD-7700, FD-7703, FD-6500, FD-6503. Correct? Likewise ST-7703. Correct? So why the blue lines? The two blue lines on the left appear to be redundant with the black lines between the larger boxes. Or do they apply only when using the 7703 as a triple, thus the cryptic footnote?

Since I have to use the FC-6503 crankset, reading right to left I must use FD-6503 and ST-6510.

FD-6603 is compatible with ST/FC-6603 and nothing else.

Yet I've had several people tell me that FD-6603 will work with the 6500-series components. Anyone have direct experience with these bits? And if they'll interoperate, care to comment on the meaning of "compatibility" in Shimano-land?

Reasoning a bit on this, from looking at the Rear chart, you see that RD-6600-GS is interchangable with RD-6500-GS in a 9-speed triple setup. Yet I understand that the front chainring spacing is the same between 9- and 10-speed systems. I would think that the 10-speed rear chainring spacing is narrower to fit another cog in the same space. The only other difference is the chain, which is common to both front and read. So wouldn't that imply less compatibility at the rear than at the front? Yet the rear is listed as compatible and the front incompatible?

Ow! My brain hurts.