The Forge Lexicon is a tool added by Thermal Foundation. It allows for the manual or automatic conversion of interchangeable blocks and items to equivalent forms, as registered in the Minecraft Forge Ore Dictionary.
Using the Forge Lexicon opens the 'Knowledge' GUI. This GUI allows players to select their preferred variations of items with equivalent forms in the Ore Dictionary.
Preferences are selected by first selecting an Ore Dictionary name in the list, then selecting the preferred variation using the arrows, and finally clicking the checkmark labeled 'Set Preference'. They can be undone by clicking the button labeled 'Clear Preference' next to the checkmark.
Ore Dictionary preferences are saved per player, not per Lexicon. Using another Forge Lexicon will display the same preferences for the same player.
The Forge Lexicon can be empowered by pressing the V key (changeable) while holding it. While empowered and in the player's inventory, the Lexicon converts any interchangeable items picked up by the player to the variations that are selected as preferences, where possible. If an interchangeable item is picked up that does not have a variation selected by the player, the item will be converted to the first in the list of variations for that item.
Using the Forge Lexicon while empowered opens the 'Transmutation' GUI. This GUI allows players to manually convert interchangeable items to the variations of their choice.
Interchangeable items are manually converted by putting an item in the slot on the left, then choosing the Ore Dictionary name to convert by using the arrows below, then choosing the variation of the item to convert to using the arrows above, and finally clicking the arrow in the middle. Converted items appear in the slot on the right, ready to be taken back by the player.
The Forge Lexicon can be reverted to the default state by pressing the V key once more.