D0–D13 = 0–13, A0–A5 = 14–19 (as on an Uno), SDA = 20, SCL = 21, LED_BUILTIN = 22.

On an Uno, 18/19 are A4/A5 and SDA/SCL. On the Phyduino Pico, SDA/SCL (GPIO24/25) are separate from A4/A5 (MAX11635), so they can't share numbers. Keeping A0–A5 contiguous at 14–19 keeps for (p = A0; p <= A5; p++) and Uno examples working. Sketches use SDA/SCL by name, so their numbers rarely matter.

The silkscreen labels D18/D19 don't match pin numbers 18/19 (those are A4/A5). This is documented on the board page.