Double Square Polyax nipples.

The Double 4-Kant (double square) head allows for tensioning of the spoke from either side of the rim. A big advantage over traditional nipples is that the wheel can be built without the visible surface of the nipple being damaged. They also allow for better alignment with the spoke and help prevent excessive buckling of the spoke while lacing.

This head style adds 4mm to the overall nipple length, so an 18mm nipple has an effective length of 14mm, and the 16mm nipple has an effective length of 12mm.
These nipples are designed to help with centring and can be used for hand lacing.