# Q&A: Why are the Segment Addition Postulate and the Partition Postulate Two Different Things?

Cross-posted from here.

I could be wrong but those two ideas sound the same, just that the partition postulate is more general.

Segment Addition Postulate: If three points A, B, and C are collinear such that B lies between A and C, then the sum of the lengths of segment AB and segment BC is equal to the length of the entire segment AC.

Partition Postulate: The whole is equal to the sum of its parts.

Without going down into the rabbit hole of axiomatic systems, I think the high-level answer you’re looking for is that

1. yes, the segment addition postulate is a more specific case of the partition postulate, and
2. the reason why a geometry course might present both postulates is that the segment addition postulate clarifies what is meant in a case where the partition postulate is vague enough to lead to ambiguity.

To elaborate on item 2 – it could be tempting to claim that the partition postulate supports the following statement, which is not true in general:

• Given three points A, B, and C, the sum of the lengths of segment AB and segment BC is equal to the length of the entire segment AC.

The segment addition postulate specifies the additional conditions that are required for the statement to hold true in general:

• Given three collinear points A, B, and C such that B lies between A and C, the sum of the lengths of segment AB and segment BC is equal to the length of the entire segment AC.

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