Ellis Briggs Fixed Gear

I don't mean cast.

Retrobike:

Pace frames are famed for the use of custom extruded and externally milled square section tubing, making them instantly recognisable. The reason for the square tubing has been explained as offering more stiffness to weight over traditional tubes. The externally milled butting allowed accurate placement of butting on the face of the tubes where it was best suited to reduce weight and not stiffness or strength.