The third level is split into 4 divisions, each with 20 teams, and these regions can vary depending on the geographical make-up of the competing teams. Rather confusingly, this is called Segunda Division B. At level four or Tercera (I know... that means third!) there are 18 divisions at regional level e.g. Galicia, Euskadi, Catalonia etc. each with 20 teams.
Promotion and relegation is a rather complex affair of play-offs which vary from division to division. Here is the current process:-
La Primera - 3 teams automatically relegated to La Segunda
La Segunda - 2 teams automatically promoted to La Primera. One team promoted through a play-off between the next four eligible teams. 4 teams automatically relegated to Segunda B.
Segunda B - 4 teams promoted to La Segunda through the play-offs. The four group winners have the opportunity to promote directly and become the overall Segunda División B champion. The four group winners are drawn into a two-legged series where the two winners will be promoted to La Segunda División (There is also a rather pointless 2-legged Segunda B champion tie!) The two losing semifinalists will enter the playoff round for the last two promotion spots.
The four group runners-up will be drawn against one of the three fourth-placed teams outside their group while the four third-placed teams will be drawn against each other in a two-legged series. The six winners will advance to the next stage and be joined by the two losing semifinalists to determine the four teams that will enter the last two-legged series for the last two promotion spots.
16 teams are automatically relegated from Segunda B (Teams finishing 17th to 20th in each of the four regional groups), along with two of the four teams that finished in 16th spot. These four teams a drawn to play in a two-legged relegation play-off, the losers dropping to the Tercera.
Tercera - 18 teams promoted to Segunda B. The eighteen group winners have the opportunity to be promoted directly to Segunda División B. The eighteen group winners were drawn into a two-legged series where the nine winners will promote to Segunda División B. The nine losing clubs will enter the play-off round for the last nine promotion spots.
The eighteen runners-up were drawn against one of the eighteen fourth-placed clubs outside their group and the eighteen third-placed clubs were drawn against one another in a two-legged series. The twenty-seven winners will advance with the nine losing clubs from the champions' series to determine the eighteen teams that will enter the last two-legged series for the last nine promotion spots.
In all the playoff series, the lower-ranked club play at home first. Whenever there is a tie in position (e.g. like the group winners in the champions' series or the third-placed teams in the first round), a draw determines the club to play at home first. Phew!
Futbolme offers the most comprehensive coverage of fixtures, results and club information and is easy to navigate. And finally, this Spanish language site lafutbolteca.com offers superb club histories for the anally retentive, just like yours truly.