  The number of regions of the type A_{n-1} Shi arrangement in R^n is counted
by the intrinsically beautiful formula (n+1)^{n-1}. First proved by Shi, this
result motivated Pak and Stanley as well as Athanasiadis and Linusson to
provide bijective proofs. We give a description of the Athanasiadis-Linusson
bijection and generalize it to a bijection between the regions of the type C_n
Shi arrangement in R^n and sequences a_1a_2...a_n, where a_i \in \{-n,
-n+1,..., -1, 0, 1,..., n-1, n\}, i \in [n]. Our bijections naturally restrict
to bijections between regions of the arrangements with a certain number of
ceilings (or floors) and sequences with a given number of distinct elements. A
special family of posets, whose antichains encode the regions of the
arrangements, play a central role in our approach.
