  Let B be an undefined quaternion algebra over Q. Following the explicit
chacterization of some Eichler orders in B given by Hashimoto, we define
explicit embeddings of these orders in some local rings of matrices; we
describe the two natural inclusions of an Eichler order of leven Nq in an
Eichler order of level N. Moreover we provide a basis for a chain of Eichler
orders in B and prove results about their intersection.
