relative pronouns - What does mean "both" in this sentence?

Ruby has two forms of multiple-selection constructs, both of which are called case expressions and both of which yield the value of the last expression evaluated.

what both refers to?

Answer

Consider the following sentences:

1. 
   

   Raj has two balls, both of which are red.

   
   


2. 
   

   Raj has two balls of equal size, both of which are red in colour.

   
   

There are two balls about which the sentence after comma describes.

So "both" in your sentence describes about the 'two forms'.

Here 'both' is used as a pronoun which refers two things together. In this case the two forms. And both always mentions two things and not multiple things or so.