In more detail
1920:1080 simplifies to 16:9 by dividing both terms by their greatest common factor, 120. Scaling keeps the ratio: a recipe in the ratio 2:3 uses 6 parts of the first for every 9 of the second. A ratio a:b splits a whole into a + b parts, so the first quantity is a / (a + b) of the total. Ratios can have more than two terms, such as concrete mixed 1:2:3 (cement, sand, gravel).
Why it matters
Ratios describe mixtures, scale drawings, screen and photo shapes (Aspect ratio), gear trains and financial measures such as debt-to-income. Order matters: 2:3 and 3:2 are different mixes. When a ratio is converted to a fraction or percentage of the whole, the denominator is the total of the parts, not the other term.
Example
split 120 in the ratio 2:3 parts = 5, one part = 24
→ 48 and 72
map scale 1:25,000 4 cm on the map = 100,000 cm = 1 km
simplify 1920:1080 GCF 120 → 16:9
In the 2:3 split, the first share is 2/5 = 40% of the total, not 2/3.
Common confusion
A Fraction compares a part with the whole, a ratio compares parts with each other. A class with 12 girls and 18 boys has a girl-to-boy ratio of 2:3, but girls are 2/5 of the class.