What does underscore(_) mean in pipes?

Question

ruby

Let's say in this code? Why does ,_ come after key?
hh.select{|key,_| params[key] == ""}

Answer 1

Placeholder for the last used variable:
stackoverflow.com/questions/9559561/where-and-how-...
The example is very strange, its meaning is not clear at all. But if you try to reproduce it:

hh = { test: 1}
params = { test: '' }
hh.select{|key,_| params[key] == ""} # { test: 1}
hh.select{|key| params[key] == ""} # { test: 1}

We will see that there is no difference. Perhaps there were some artifacts in the old (1.8, 1.9) versions of the interpreters.