  We introduce the new concept of D-geometry (or "drum geometry"), which has
been recently discovered by the author in \cite{KT-DRUMS} when constructing and
classifying isospectral and length equivalent drums under certain constraints.
We will show that any pair of length equivalent domains, and in particular any
pair of isospectral domains (which makes one unable to "hear the shape of
drums") which is constructed by the famous Gassmann-Sunada method, naturally
defines a D-geometry, and that each D-geometry gives rise to such domains. One
goal of this letter is to show that in the present theory of isospectral and
length equivalent drums, many examples are controlled by finite geometrical
phenomena in a very precise sense.
