Basic SQL join question

Sorry for this simple question but I can't seem to get Postgres to do 
what I want ...

I want to get the concatenation of 2 or more tables with absolutely 
nothing in common. How can I do this?

For example

Table a:

   a
-----
  a1
  a2
  a3

Table b:

   b
-----
  b1
  b2

Table c:

   c
-----
  c1
  c2
  c3
  c4

What is the proper SQL to return:

   a |  b |  c
---------------
  a1   b1   c1
  a2   b2   c2
  a3        c3
            c4


Thanks,

Jc