diff --git a/src/main/java/io/github/homchom/recode/game/SlotConversions.kt b/src/main/java/io/github/homchom/recode/game/SlotConversions.kt
new file mode 100644
index 00000000..13fc8ae7
--- /dev/null
+++ b/src/main/java/io/github/homchom/recode/game/SlotConversions.kt
@@ -0,0 +1,120 @@
+package io.github.homchom.recode.game
+
+import io.github.homchom.recode.util.InvokableWrapper
+
+/**
+ * An index of a [net.minecraft.world.Container] slot, ordered by [order]. The order is stored in type
+ * parameter @T and can be changed, allowing for safe slot conversions.
+ *
+ * @see SlotOrder
+ */
+data class SlotIndex<out T : SlotOrder>(
+    override val value: Int,
+    private val order: T
+) : InvokableWrapper<Int> {
+    /**
+     * Converts the index to one in order [order].
+     */
+    fun <S : SlotOrder> reorder(order: S) = converted(value, this.order, order)
+
+    companion object {
+        /**
+         * @return A [SlotIndex] with order [targetOrder] and an index equivalent to [value] in [sourceOrder].
+         */
+        fun <S : SlotOrder> converted(value: Int, sourceOrder: SlotOrder, targetOrder: S): SlotIndex<S> {
+            val (order, subIndex) = sourceOrder.getOrNull(value)
+                ?: throw IndexOutOfBoundsException("Slot order $sourceOrder does not have index $value")
+
+            val newIndex = targetOrder.indexOf(order, subIndex)
+            require(newIndex != -1) {
+                "Slot order $targetOrder does not have an index equivalent to $value in $sourceOrder"
+            }
+
+            return SlotIndex(newIndex, targetOrder)
+        }
+    }
+}
+
+/**
+ * A recursive total ordering of [net.minecraft.world.Container] indices with identity.
+ *
+ * @see SlotIndex
+ */
+sealed interface SlotOrder { // TODO: add unit tests
+    data object Hotbar : SlotOrder by slots(9)
+    data object UpperInventory : SlotOrder by slots(27)
+    data object Armor : SlotOrder by slots(4)
+    data object Offhand : SlotOrder by slots(1)
+    data object QuickCraft : SlotOrder by slots(5)
+
+    data object InventoryItems : SlotOrder by slots(Hotbar, UpperInventory, Armor.reversed, Offhand)
+    data object InventoryMenu : SlotOrder by slots(QuickCraft, Armor, UpperInventory, Hotbar, Offhand)
+
+    /**
+     * The identity of the order, used for equality checks.
+     */
+    val id get() = this
+
+    /**
+     * The number of slots present in this order.
+     */
+    val size: Int
+
+    /**
+     * @return The base [SlotOrder] found at [index], paired with the sub-index of [index] in it. Returns
+     * `null` if [index] is out of bounds.
+     */
+    fun getOrNull(index: Int): Pair<SlotOrder, Int>?
+
+    /**
+     * @return The index of the given nested [order]'s [subIndex] in this order.
+     */
+    fun indexOf(order: SlotOrder, subIndex: Int): Int
+}
+
+private fun slots(size: Int) = SlotRange(size)
+private fun slots(vararg compartments: SlotOrder) = SlotSequence(*compartments)
+private val SlotOrder.reversed get() = ReversedSlotOrder(this)
+
+private class SlotRange(override val size: Int) : SlotOrder {
+    override fun getOrNull(index: Int) =
+        if (index in 0..<size) id to index else null
+
+    override fun indexOf(order: SlotOrder, subIndex: Int) =
+        if (id == order.id) subIndex else -1
+}
+
+private class SlotSequence(vararg compartments: SlotOrder) : SlotOrder {
+    override val size = compartments.sumOf(SlotOrder::size)
+
+    private val compartments = compartments.toList()
+
+    override fun getOrNull(index: Int): Pair<SlotOrder, Int>? {
+        var mutIndex = index
+        for (compartment in compartments) {
+            compartment.getOrNull(mutIndex)?.let { return it }
+            mutIndex -= compartment.size
+        }
+        return null
+    }
+
+    override fun indexOf(order: SlotOrder, subIndex: Int): Int {
+        var index = 0
+        for (compartment in compartments) {
+            val nestedIndex = compartment.indexOf(order, subIndex)
+            if (nestedIndex != -1) return index + nestedIndex
+            index += compartment.size
+        }
+        return -1
+    }
+}
+
+private class ReversedSlotOrder(private val delegate: SlotOrder) : SlotOrder {
+    override val size get() = delegate.size
+
+    override fun getOrNull(index: Int) =
+        delegate.getOrNull(size - 1 - index)
+
+    override fun indexOf(order: SlotOrder, subIndex: Int) =
+        delegate.indexOf(order, order.size - 1 - subIndex)
+}
\ No newline at end of file
diff --git a/src/main/java/io/github/homchom/recode/hypercube/state/Permissions.kt b/src/main/java/io/github/homchom/recode/hypercube/state/Permissions.kt
index 8f8bcf4f..9d905984 100644
--- a/src/main/java/io/github/homchom/recode/hypercube/state/Permissions.kt
+++ b/src/main/java/io/github/homchom/recode/hypercube/state/Permissions.kt
@@ -3,6 +3,7 @@ package io.github.homchom.recode.hypercube.state
 class PermissionGroup(ranks: Iterable<Rank>) {
     val ranks = ranks.toSet()
 
-    inline operator fun <reified P> contains(rankPermission: P) where P : Rank, P : Comparable<P> =
+    inline operator fun <reified P> contains(rankPermission: P)
+    where P : Rank, P : Comparable<P> =
         ranks.any { it is P && it >= rankPermission }
 }
\ No newline at end of file
diff --git a/src/main/java/io/github/homchom/recode/multiplayer/PlayerExtensions.kt b/src/main/java/io/github/homchom/recode/multiplayer/PlayerExtensions.kt
index 92619da8..4e639313 100644
--- a/src/main/java/io/github/homchom/recode/multiplayer/PlayerExtensions.kt
+++ b/src/main/java/io/github/homchom/recode/multiplayer/PlayerExtensions.kt
@@ -1,5 +1,43 @@
 package io.github.homchom.recode.multiplayer
 
+import io.github.homchom.recode.game.SlotIndex
+import io.github.homchom.recode.game.SlotOrder
+import io.github.homchom.recode.mc
+import net.minecraft.client.player.LocalPlayer
+import net.minecraft.world.entity.player.Inventory
 import net.minecraft.world.entity.player.Player
+import net.minecraft.world.item.ItemStack
 
-val Player.username: String get() = gameProfile.name
\ No newline at end of file
+val Player.username: String get() = gameProfile.name
+
+/**
+ * @return A [Sequence] of this [Inventory]'s items in the order specified by [SlotOrder.InventoryItems].
+ */
+fun Inventory.asSequence() = sequenceOf(items, armor, offhand).flatten()
+
+/**
+ * Sets this player's creative [Inventory] [slot] to [stack] and synchronizes it with the server.
+ *
+ * @throws IllegalStateException if the player is not in creative mode.
+ */
+fun LocalPlayer.setItemAndSync(slot: SlotIndex<SlotOrder.InventoryItems>, stack: ItemStack) =
+    setItemAndSync(slot, slot.reorder(SlotOrder.InventoryMenu), stack)
+
+/**
+ * Sets this player's creative [Inventory] [slot] to [stack] and synchronizes it with the server.
+ *
+ * @throws IllegalStateException if the player is not in creative mode.
+ */
+@JvmName("setItemAndSyncMenu")
+fun LocalPlayer.setItemAndSync(slot: SlotIndex<SlotOrder.InventoryMenu>, stack: ItemStack) =
+    setItemAndSync(slot.reorder(SlotOrder.InventoryItems), slot, stack)
+
+private fun LocalPlayer.setItemAndSync(
+    itemsSlot: SlotIndex<SlotOrder.InventoryItems>,
+    menuSlot: SlotIndex<SlotOrder.InventoryMenu>,
+    stack: ItemStack
+) {
+    check(isCreative) { "Inventory items can only be set by the client in creative mode" }
+    inventory.setItem(itemsSlot(), stack)
+    mc.gameMode?.handleCreativeModeItemAdd(stack, menuSlot())
+}
\ No newline at end of file