  Most investigations of multipartite entanglement have been concerned with
temporal modes of the electromagnetic field, and have neglected its spatial
structure. We present a simple model which allows to generate tripartite
entanglement between spatial modes by parametric down-conversion with two
symmetrically-tilted plane waves serving as a pump. The characteristics of this
entanglement are investigated. We also discuss the generalization of our scheme
to 2N+1-partite entanglement using 2N symmetrically-tilted plane pump waves.
Another interesting feature is the possibility of entanglement localization in
just two spatial modes.
