Hairline Rule переводится на русский язык как "линейка для волос".

Категория: не задано

In typography, a very thin rule line typically less than one-half point wide. On some output devices, the hairline rule is as thin as the smallest printer spot the device can image. On 600 ppi laser printers, the hairline rule is effective; however, on high-resolution (2400+ ppi) imagesetters, it can be essentially invisible. See Rule Line.