Final answer:
The ruler placement postulate states that when using a ruler to measure a line segment, the first mark of the ruler should be aligned with one endpoint of the line segment, and the second mark of the ruler should be aligned with the other endpoint.
Step-by-step explanation:
The ruler placement postulate states that when using a ruler to measure a line segment, the first mark of the ruler should be aligned with one endpoint of the line segment, and the second mark of the ruler should be aligned with the other endpoint of the line segment. The length of the line segment is then read from the ruler using the units marked on it, such as inches, centimeters, or millimeters.
For example, if you are measuring a line segment using a ruler with centimeters as the unit, you would align the 0 cm mark of the ruler with one endpoint of the line segment and the appropriate other mark (let's say the 10 cm mark) with the other endpoint. Then, you would read the length of the line segment as 10 cm from the ruler.