For many of the Arduinos, the Digital pins 0 through 13 are space 0.1 inches apart from their neighbor. That is, except for pins 7 and 8. They are 0.15 inches apart. For this reason, you can not easily make make your own shields using standard perf boards.
The pins on the other side of the board do not have this problem. A0 and Vin are 0.2 inches apart. Also A5 lines up with D0 and are rows of pins are spaced 1.9 inches apart so they line up nicely. D8-D13,Gnd and AREF are the only problem pins.
What's the deal?