Line engraving is a subtractive art. The plate is polished flat, the design is drawn backwards on it, and then the burin — a hardened steel chisel — is pushed along the line, throwing a curl of copper ahead of itself. Everything the printer ever shows is the shadow of a groove.
Because the cut has width and because the paper takes ink unevenly, a single engraved line is never one line. It is a dark core with a soft shoulder on either side, and that shoulder is what gives copperplate its particular darkness. This plate is drawn with that in mind: every stroke is paired with a hairline beside it.