A sign assembly have a chassis assembly for supporting indicia plates and for providing proper horizontal alignment of the indicia plates. The chassis assembly includes two chassis members having a medial receptacle for receipt of a support panel and a receptacle for receipt of alignment members. The indicia plates include mating members on distal edges for cooperating with the chassis member alignment members. The mating members and alignment members have cooperating surface configurations to facilitate proper indicia plate horizontal alignment. A tamper resistant assembly prevents unintentional removal of the indicia plates from the chassis assembly.