Numbers can be positive or negative. On a horizontal number line, positive numbers are to the right of 0 and negative numbers are to the left of 0. We read "-3" as "negative 3". A positive number can be written with a "+" sign (like +4) or without one (like 4).

The number line tells us how numbers compare. The greater number is the one farther to the right, and the less number is the one farther to the left. For example, -2 is greater than -5 because -2 is to the right of -5, and -5 is less than -2 because -5 is to the left of -2. Any positive number is greater than any negative number.

Some numbers are closer to zero than others. To find out, we count how many units each number is away from 0. For example, -10 is closer to zero than -35 because -10 is only 10 units from 0, while -35 is 35 units from 0.

Number lines do not have to go left to right. In real life, many number lines go up and down. A thermometer is a vertical number line for temperature: higher on the thermometer means warmer. Below 0° is cold (negative), above 0° is warm (positive).

Elevation is another vertical number line. Sea level is 0 meters. A bird in the sky has a positive elevation (above 0). A fish swimming below the ocean surface has a negative elevation (below 0). An elevation closer to 0 means closer to sea level.