perpendicular – (adjective) at a right angle with (90 degrees)

perpendicularly – (adverb) being at a right angle with (90 degrees)

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Examples:

The roof is perpendicular to the two columns.

Those two lines aren’t perpendicular, they’re parallel to each other.

The cars in the photo on the left-hand side are parked perpendicularly.

When the line of a fishing rod isn’t hanging perpendicularly, it may mean a fish is pulling at your bait.