We have also A0, A1, ..., A5 markings on the NANO Board. Why?
We need to know that:
The marks D0 - D13 marks refer to 0 (Digital Pin-0) - 13 (Digital Pin-13) respectively.
The A0 - A5 marks refer to 14 (Digital Pin-14) - 19 (Digital Pin-19) respectively.
The Arduino Compiler knows the meanings of the digital pins as --
0 - 7 for PD0 - PD7 (Port Pins of PORT D of the MCU)
8 - 13 for PB0 - PB5 (Port Pins of PORT B)
14 - 19 as PC0 - PC5 (Port Pins of PORT C)
The following diagram may be helpful:
