[mikedc]

Users agree to a EULA which stipulates how they can and can't use the font. This contract provides the legal basis on which a foundry would pursue someone for suspected misuse. Here's Monotype's EULA for Helvetica Now[0], Section 9 specifically addresses copies and derivative works. From there, it becomes a legal matter.

[0] https://www.fonts.com/font/monotype/helvetica-now/licenses#

[alberth]

But how can you differentiate a Helvetica created font file from a font file where I manually traced every Helvetica alphabet identically.

[addaon]

Font cloning is a thing.

Font tracing is usually done by printing out the character to be traced at very large scale -- I've seen about 12" x 12" -- and placing it directly on a large digitizing tablet. A sequence of strokes / points is collected for the outline of the character, and then curves of somewhat reduced degree are fit to those strokes / points to both reduce font data size and reduce the impact of errors, inaccuracies, and quantization in the data capture.

Even at this huge scale, and with this amount of effort, the outline of your character will be very close to -- visually identical to! -- the starting character, but not exact. As a result, the generated font program will be quite different. For example, it may use a different number of control points for equivalent curves.

Now, one can imagine automating this process differently: Take a font file, digitally render each character, perturb it a small amount, and resynthesize the strokes to generate a new, different program for a visually identical font. This is generally against the terms of service for the initial font, however, which would make it a legal matter...

[jjeaff]

I suppose if you manually traced Helvetica letters, then they might have a case for misusing their font. After all, there isn't a licensing option that allows you to use the font as a template.

[xsmasher]

If you copied the .ttf file and sold it, it's a copyright violation just like any software copying. The foundry takes you to court.

If you copy by hand (at what size? at what accuracy? do you include the same hinting and ligatures?) the file will not be bit-for-bit identical. The foundry cannot sue.