Synchronic and diachronic axes
Language that is studied synchronically is "studied as a complete
system at a given point in time" (The AB axis). Language studied diachronically is "studied in its historical development" (The CD axis).
Saussure argues that we should be concerned with the AB axis (in addition to
the CD axis, which was the focus of attention in Saussure s time), because, he
says, language is "a system of pure values which are determined by nothing
except the momentary arrangements of its terms". We could study chess
diachronically (how the rules change through time) or synchronically (the
actual rules).
To illustrate this, Saussure uses a chess metaphor. In chess, a person joining a game’s
audience mid-way through requires no more information than the present layout
of pieces on the board and who the next player is. They would not benefit from
knowing how the pieces came to be arranged in this way.