Each weight and style comes in three different grades. Grades are similar to weights, except that the differences in width between grades are very subtle. Each font comes in three grades, Regular (-), Regular and Regular (+) which are basically lighter and darker versions of the weight using the same metrics information. This is a further difference between weight and grade; when setting text with different font grades, the widths remain the same, so the text doesn’t reflow.
Grades are also useful when combining positive and negative text (light text on dark background), and trying to achieve even typographic ‘colour’ of the text. Because the white reflects more light than dark colours, it is difficult to get the same appearance of the text on light background as in dark ones.
Reproduction technology effects the choice of the grades. When negative text is used on a computer screen, backlight is used, and a grade lighter fonts are recommended. However in print, one might need to compensate for the dot gain* by choosing darker grade (+) of the font when setting the text in negative.
Finally, using different grades of the font is useful when combining the text in different sizes, while trying to optically balance the appearance of the text. A smaller text requires darker grade of the font than larger text. Example is using (+) grades for footnotes, and (-) grade for the body text.
The grades are available only as part of the complete package and cannot be purchased separately.*Dot Gain (or Tonal Value Increase) is the term that describes the spread of ink on paper, causing the dots and lines which make up the image to print larger than they should. The result is darkening of images and text.
Grades help fine-tuning typographic colour of the text.
Using (-) grades helps to compensate back light spilling on a computer screen.
Using (+) grades to compensate for spreading of the ink on paper, when setting white text on black background.