A number line is a straight line with numbers marked at regular intervals, used to show how numbers relate to each other in space. Zero usually sits in the middle, with positive numbers stretching to the right and negative numbers to the left. Each mark represents a specific value, and the distance between marks stays consistent.

Picture a long ruler lying flat. At the center, you mark zero. To the right, you mark 1, 2, 3, and so on. To the left, you mark -1, -2, -3, continuing as far as you need. The number line helps you visualize math operations: adding means moving right, subtracting means moving left.

Number lines appear everywhere in mathematics. They help explain concepts like greater than and less than (numbers farther right are bigger). They show how fractions fit between whole numbers. They make negative numbers less mysterious by giving them a clear location. On a number line, you can see that -5 is colder than -2 because it sits farther left, farther from zero.

Teachers often draw number lines on whiteboards to demonstrate problems. Thermometers are vertical number lines showing temperature. Even though numbers continue forever in both directions, we usually draw just the portion we need for whatever problem we're solving.