Full Idea
The C-series sees times not as directed, but as unchanging, and ordered in terms of the betweenness relation. The C-theory also asserts that the A-series and B-series do not exist.
Gist of Idea
The C-series rejects A and B, and just sees times as order by betweenness, without direction
Source
Baron,S/Miller,K (Intro to the Philosophy of Time [2019], 1.2)
Book Reference
Baron,S/Miller,K: 'Introduction to the Philosophy of Time' [Polity 2019], p.12
A Reaction
This is McTaggart's idea. Compare this with A-series ordering by past, present and future, and B-series ordering by earlier-than, later-than and simultaneous. The main point is that A and B have a direction, but C does not.