Extending in scope or effect to a prior time or to conditions that existed or originated in the past; especially : made effective as of a date prior to enactment, promulgation, or imposition
The main purpose of subheadings are: They stand out because of their size and attract attention. The scanner will stop to read them and continue scanning until the next subhead which they'll then read. Scanning from subhead to subhead, they serve to guide the reader down the page.
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Hope it helps...</h2>
