Cascade Summon

A Cascade Summon ( Kasukēdo Shōkan) is the act of Special Summoning a Cascade Monster from the Extra Deck using a Cascade Spell Card and two or more monsters with consecutive Levels.

How to Cascade Summon
During their Main Phase, if the player controls a cached Cascade Spell Card, the turn player can Cascade Summon a Cascade Monster from their Extra Deck by first activating the Cascade Spell Card (turning it vertically from being sideways), then using two or more monsters with consecutive Levels as Cascade Materials, whose Levels either approach (from above or below) or include the Class of the Cascade Monster being Summoned, that are listed on it as materials.

Cascade Materials whose Levels are lower than or equal to the Cascade Monster's Class are sent to the Graveyard, while materials with a Level higher than the Class are attached to the Cascade Spell Card used in the Cascade Summon. If all of the Cascade Materials used in the Cascade Summon have lower Levels than the Cascade Monster's Class (approaching from below), the Cascade Spell Card is also sent to the Graveyard.

After these actions are performed, the player Summons the Cascade Monster from their Extra Deck to any of their unoccupied Monster Zones in face-up Attack or Defense Position. The Summoned Cascade Monster is assigned a Limit equal to the highest Level among the Cascade Materials used in its Summon, minus the Class of the Cascade Monster, plus 1. Put another way, the Limit of the Cascade Monster is equal to the number of cards that are not sent to the Graveyard as part of the Cascade Summon, among the Cascade Spell Card and the Cascade Materials. For example, for a two-material Cascade Summon, three cards are involved: the materials, and the Cascade Spell itself. If the Cascade Spell remains on the field after the Summon, and exactly 1 monster is attached to it as a material, then we can count both the Cascade Spell (+1) and the attached monster (+1) as cards not sent to the Graveyard, thus giving a Limit of 2.

Even if all of its controller's Monster Zones are occupied, a Cascade Monster can still be Cascade Summoned as long as at least one of the Cascade Materials used is in one of these zones, so that there is an available zone when the Cascade Monster is Summoned.

Example
The Level 3 and Level 4 monsters are sent to the Graveyard, as well as the Cascade Spell Card, thus 0 cards used in the Summon are not sent to the Graveyard, hence a Limit of 0.
 * A Level 3 monster + A Level 4 monster = A Class 5, Limit 0 Cascade Monster (4 - 5 + 1 = 0).

The Level 4 and Level 5 monsters are sent to the Graveyard. The Cascade Spell Card remains on the field, thus exactly 1 card used in the Summon does not get sent to the Graveyard, hence a Limit of 1.
 * A Level 4 monster + A Level 5 monster = A Class 5, Limit 1 Cascade Monster (5 - 5 + 1 = 1).

The Level 5 monster is sent to the Graveyard. The Cascade Spell Card remains on the field, and the Level 6 monster is attached to it as a material, thus exactly 2 cards used in the Summon are not sent to the Graveyard, hence a Limit of 2.
 * A Level 5 monster + A Level 6 monster = A Class 5, Limit 2 Cascade Monster (6 - 5 + 1 = 2). 

No cards are sent to the Graveyard. The Cascade Spell Card remains on the field, and the Level 6 and Level 7 monsters are attached to it as a material, thus exactly 3 cards used in the Summon are not sent to the Graveyard, hence a Limit of 3.
 * A Level 6 monster + A Level 7 monster = A Class 5, Limit 3 Cascade Monster (7 - 5 + 1 = 3).

Advanced rulings

 * A Cascade Summon is a kind of built-in Special Summon, like Contact Fusion, Pendulum Summon, Synchro Summon or the Special Summon of a monster like "Black Luster Soldier - Envoy of the Beginning", therefore it does not start a Chain.
 * If the activation of a cached Cascade Spell Card is negated, the Cascade Summon will not occur as the activation must succeed to proceed with using the materials. However, this does not count as negating or responding to the Cascade Summon, but rather as negating/responding to the Cascade Spell Card's activation.
 * Cascade Materials currently attached to a Cascade Spell are not treated as being on the field, but are still treated as monsters, unlike Xyz Materials, and thus can be affected by cards such as "Dimensional Fissure".
 * Token Monsters can be used as Cascade Materials if their Levels are lower than or equal to the Cascade Monster's Class, but not if their Levels are higher, as Tokens cannot be in any state other than being on the field.
 * Unlike Xyz Materials, Cascade Materials can only be monsters; never Spell or Trap Cards, as they are designed to be Special Summoned from them. Card effects cannot attach Spell or Trap Cards to a Cascade Spell Card.
 * As a consequence of not being able to attach Spell or Trap Cards, Trap Monsters cannot be used as Cascade Materials if their Levels would exceed the Class of the Cascade Monster that would be Cascade Summoned, as Trap Monsters are originally Trap Cards, not Monster Cards, and thus would be an attempt to attach a non-Monster Card to a Cascade Spell Card. However, like Tokens, Trap Monsters may still be used as materials provided their Levels are lower than or equal to the Cascade Monster's Class.
 * A Cascade Monster that is Special Summoned by a method other than a Cascade Summon has a Limit of 0, unless a card effect states otherwise.
 * If a Cascade Monster with a Limit of 1 or higher is flipped face-down, then later flipped face-up again, its Limit becomes 0.

Trivia

 * Using monsters as Cascade Material to perform a Cascade Summon is referred to as entangling them, just like how using monsters as Synchro Material is referred to as tuning them and using monsters as Xyz Material is known as overlaying.