The Arch of Titus, Rome Italy.
  The Arch d'Triomphe, Paris, France.
  The Mostar bridge in the former Yugoslavia.
  Nicolas Janberg's Structurae is an excellent site for exploring arch and many other types of structures.
By pushing inwards at the bottom of the arch, we counteract the arch thrust force and so we say that the arch is stable.  I've provided links to some representative examples of arches that you might find interesting.

Once the fabrication and assembly problems are overcome, the arch makes a durable structural component.  So how did the Romans use arches in their structures?  We'll see beginning on the next page.