The Study of the Elements: Spell Trees
In the Sister Continents those folks who channel spells in one manner or another will study their art following what are called Spell Trees. The Spell Tree is composed of a core Trunk of spells from which Branch numerous additional spells. At the base of the Trunk are the easiest spells to learn and use. Most Trees will start at Tier 1 (with the amount of Elemental Energy required to cast the spell equaling the Tier) but there are some high level exceptions where spells will start higher and have additional prerequisites in order to learn the Tree. These Trees still follow most of the basic rules laid out below.
Spell Trees always have a theme, no matter which type of Channeling is used to harness the Elemental Energies. In the case of Sorcery, Trees are almost exclusively themed to a single Element and all spells on the Tree will have that Element as their primary source of energy, while in the case of Holy Magic, the theme may spread across several Elements, such as a Tree with a Barriers theme. Mentalists tend to fall in the middle of these two extremes, with Trees often dealing with Elemental Life and Spirit. Example Spell Tree.
When a Character successfully trains a Spell Tree they are automatically proficient with the Tier 1 spell, but they are not entirely limited to that single spell, even at first level. (see: Overcasting in the Mechanics Section) Basically speaking, when a Character learns that first spell, they have a basic grasp of the theme and how to manipulate it, and therefore they are able to attempt to Overcast their Proficiency with any spell on the Trunk†. This is clearly a dangerous proposition if the attempt is too over-reaching or repeated.
In order to cast spells from the First and Second Branches of the Tree the Character must be Specialized and Double Specialized in that Tree and perform Spell Study in order to learn these spells. In order to study a Second Branch Spell, the Character must also be Proficient in the First Branch connected to that spell.
In addition to these spells there are occasionally Research or Third Branch spells. These spells require additional time and effort in order to learn, and also lay the groundwork as examples for how the Spell Research rules function, where Characters will have the option to Research spells of their own devising.
† - Rules variations, particularly applying to magic technology and schools of magic, will alter and limit this base rule.