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  • Bill Levien thinks this is interesting:

The top and bottom rows (coloured in red and blue and marked with + and –) are connected horizontally and are used to carry the power across the board so that when you need power or ground, you can provide it very quickly with a jumper (a short piece of wire used to connect two points in the circuits).

From

Cover of Getting Started with Arduino, 2nd Edition

Note

When wiring up a pull-down resister/button and it doesn't work, CHECK THAT YOU GROUNDED BOTH NEGATIVE BUSES!