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Ratio is the relationship between two numbers, lines or quantities. Comparing two numbers together forms a ratio, example: 2 is to 4 is expressed as 2:4. Reviewing the 2:4 ratio we notice 4 is double 2 and you could also say 2 is to 4 as 3 is to 6 (3:6). Thus 2:4 and 3:6 are have same ratio. Likewise 3:9 is the same as 4:12.

Proportion combines two equal ratios. Thus we say 6 is to 2, as 9 is to 3 because 6 is three times greater that 2, and 9 is three times greater that 3. It is the same with regard to lines. Thus a line 60mm long has the same proportion to a line 20mm long as compared to a line 90mm and 30mmm long.