The Nano and MKR footprints

Except for the Arduino Zero, which is in the traditional Arduino Uno R3 footprint, all other ARM-based Arduino boards are in either the Nano or the MKR footprints. Apart from size, what sets these apart is the type of connector used for the pins: instead of sockets mounted on the top of the board, as was the case with the Arduino Uno, the Nano and MKR footprints have pins that are mounted on the bottom of the board, making them suitable to plug directly into a breadboard. Similarly, whereas shields sit on top of an Uno, shields for MKR or Nano boards sit underneath the microcontroller board.