Each advanced class is derived from either the Squire or Chemist class. The Squire class leads you toward stronger warrior classes, while the Chemist class will take you more towards magic and other forms of battle.

Here is a quick reference for jobs and their prerequisites:

Squire	None
Knight	Level 2 Squire
Archer	Level 2 Squire
Monk	Level 2 Knight
Thief	Level 2 Archer
Geomancer	Level 3 Monk
Lancer	Level 3 Thief
Dancer (females only)	Level 4 Geomancer, Level 4 Lancer
Chemist	None
Priest	Level 2 Chemist
Wizard	Level 2 Chemist
Oracle	Level 2 Priest
Time Mage	Level 2 Wizard
Mediator	Level 2 Oracle
Summoner	Level 2 Time Mage
Bard (males only)	Level 4 Mediator, Level 4 Summoner
Samurai	Level 3 Knight, Level 4 Monk, Level 2 Lancer
Ninja	Level 3 Archer, Level 4 Thief, Level 2 Geomancer
Calculator	Level 4 Priest, Level 4 Wizard, Level 3 Time Mage, Level 3 Oracle
Mime	Level 8 Squire, Level 8 Chemist, Level 4 Geomancer, Level 4 Lancer, Level 4 Mediator, Level 4 Summoner