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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).


Cover of Getting Started with Arduino, 2nd Edition


PSA: Always make sure you ground BOTH negative rails before debugging your code!