Helvetica (classified as a "Grotesque", "Neo-Grotesque", "Static", or "Rational" sans serif) is more distinct from the other two because it is has a much more "machined" appearance, with horizontal terminals, closed apertures, and very normalized lettershapes and widths. In addition, Helvetica's round shapes are more square than Myriad and Frutiger, its x-height is larger, and its ascenders are shorter.
As typefaces in the "Humanist" category, Frutiger and Myriad have much more in common with each other than with Helvetica. Their lettershapes are more closely related to the gentle curves of handwriting. Frutiger has a slightly crisper feel, partly because its terminals are sheared nearly vertically, whereas Myriad's terminals end perpendicular to the stroke direction.More questions on Typefaces:
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