A number line shows fractions as distances from 0. Fractions to the right are greater. Number lines help students see that 1/2 = 2/4 and that fractions can be compared by position.
You already know how to place fractions on a number line: divide the segment from 0 to 1 into equal parts, then count parts from the left. A fraction like 3/4 sits three-quarters of the way from 0 to 1. The number line brings something powerful that fraction circles alone cannot provide: position encodes size. Whatever is further to the right is greater. This is the same rule you've used for whole numbers, and it works for fractions too.
To compare two fractions using a number line, you just need to see which fraction is further from 0. Because 3/4 sits to the right of 1/2, you can read directly that 3/4 > 1/2 — no additional reasoning required. The number line also makes visible something that can be hard to see from symbols alone: 1/2 and 2/4 land on exactly the same point. Two fractions that occupy the same position on the number line must be equal — that's the foundation of equivalent fractions.
The key to comparing fractions on a number line is that both fractions must be measured against the same-sized whole. You can only directly compare 3/4 and 2/4 on a number line whose segment from 0 to 1 is divided into fourths. If one line is divided into fourths and another into thirds, comparing positions across the two diagrams will give misleading results. This is why mathematicians insist that fractions are only comparable when they share the same whole.
Comparing unit fractions — fractions with numerator 1 — reveals a counterintuitive pattern: 1/4 is smaller than 1/2, even though 4 is bigger than 2. On a number line, dividing the same segment into more pieces makes each piece shorter, so 1/4 reaches only a quarter of the way to 1 while 1/2 reaches halfway. The bigger the denominator, the more the whole is sliced up, and the shorter each slice. Seeing this directly on the number line — the slices literally shrinking as you divide into more parts — is the clearest way to build lasting intuition for why larger denominators mean smaller pieces.