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Classification

Quick Guide to the Slopes of Lines

A line's slope can be classified into one of four categories based on the line's direction when read from left to right:

  • Positive slope: The line rises from left to right.
  • Negative slope: The line falls from left to right.
  • Zero slope: The line is horizontal — it neither rises nor falls. Its equation has the form y=by = b.
  • Undefined slope: The line is vertical — it does not run left or right. Its equation has the form x=ax = a.

Lines are always "read" from left to right, just as written words are read in English. This four-part classification provides a quick visual check: glancing at a line's direction immediately narrows down whether its slope is positive, negative, zero, or undefined.

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Updated 2026-04-21

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