What is the meaning of open proportion?

A proportion is the statement that two ratios are equal. For example: 1/4 = 2/8. An open proportion is when you use an unknowm amount in one of the ratios, for example: 1/2 = x/10. It is open because the value of x is not fixed in advance. Of course you can solve the equality for x and you will find x = 1*10/2 = 5. Then an open prortion is the equality of two ratios where one of the terms is unknwon and represented by a variable.