The symmetry of standing wave patterns on the surface of a vertically
oscillated fluid layer depends on the depth of the layer. In a large e
xperiment we trace out the stability diagram. The dependency can be un
derstood on the basis of the oscillatory character of the waves and th
e dispersion relation. A simple amplitude equation can be constructed
with the variation with depth of the dispersion relation and a general
quadratic nonlinearity as its only ingredients. The predictions of th
is model agree well with the experiment.