What is the difference between the Mentions in time and Reach in time charts?

Mentions in time shows you the exact number of mentions across a given timespan.

Reach in time weighs the number of mentions with their reach, presenting an estimated number of impressions.

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Example

If an event generates a high number of comments, it will cause a peak on the mentions in time chart. This doesn't necessarily mean a corresponding peak on the reach in time chart — that will only happen if the mentions generate a high enough reach. Conversely, a single mention can generate an enormous reach spike if it goes viral.