.</li>
</ul><p>Plural forms are also acceptable.</p>
<p>The direction of positive, whether up or down, cannot in all cases be inferred from the units. The direction of positive is useful for applications displaying the data. For this reason the new attribute <cite>positive</cite> is defined in this standard. The inclusion of the <cite>positive</cite> attribute is required by this standard if the vertical axis units are not a valid unit of pressure (a determination which can be made using the udunits routine