  We investigate the internal observability of the wave equation with Dirichlet
boundary conditions in a triangular domain. More precisely, the domain taken
into exam is the half of the equilateral triangle. Our approach is based on
Fourier analysis and on tessellation theory: by means of a suitable tiling of
the rectangle, we extend earlier observability results in the rectangle to the
case of a triangular domain. The paper includes a general result relating
problems in general domains to their tiles, and a discussion of the triangular
case. As an application, we provide an estimation of the observation time when
the observed domain is composed by three strips with a common side to the edges
of the triangle.
