We have classified type to enhance the ability to select type. We know intutitively when type is the same or different, but we do not always know what the difference is or how to correct it.
By learning about the different classes of type we can recognise them and use this knowledge to select type reasonably.
What we have been examining is the structure of type. That is comparing:
In addition to the structure of type, there is also the shape of the letters. This also distinguishes type. This is largely what distinguishes one sans-serif type from another sans-serif. Also the capital and lower-case letters are of the same structure in any type face, but differ in shape: The G is a different shape from the g! But the same structure.
Essentially, two similar types on the same page look like a near-miss. When they are obviously different then they are at least intended rather than being a mistake. Therefore, different type should not merely be different, but be obviously different.
We can distinguish type in other ways than by structure. For example:
In the same type face, the structure is the same, but the type looks different when it is bold, italic or of a different size.
We can therefore use only one type face on a page and still get the required contrast, by using size, bold and italic.
Merely using different type faces might not give us sufficient contrast, so we would increase the contrast by changing size, etc.
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