When I install a gem, it gets installed in a directory named 1.9.1, despite that not being the version of Ruby I have installed:
$ ruby -v
ruby 1.9.3p327 (2012-11-10 revision 37606) [x86_64-darwin12.2.0]
$ gem which rails
.../ruby/gems/1.9.1/gems/railties-3.2.9/lib/rails.rb
Why does this happen? I have no other Ruby versions installed (and certainly not v1.9.1).