Let G = (V,E) be a graph. By directional labeling (or d-labeling) of an edge x = uv of G by an ordered n-tuple (a1, a2, · · · , an), we mean a labeling of the edge x such that we consider the label on uv as (a1, a2, · · · , an) in the direction from u to v, and the label on x as (an, an−1, · · · , a1) in the direction from v to u. In this survey, we study graphs, called (n, d)-sigraphs, in whic...