Stacking

Stacking is a gameplay concept in the Metroid series, which allows Samus Aran to retain the previous effects of any ability and use them in conjunction with the most recently collected upgrade.
Throughout the games, this has applied to numerous upgrades of the Power Suit. The two earliest games, Metroid and Metroid II: Return of Samus had little to no stacking, while Super Metroid was the game that introduced it. It has been present in every game since except for Metroid Prime and Metroid Prime 2: Echoes (not counting Suit upgrades), and Metroid Prime Hunters.
==Beam upgrades==
In Metroid, the Long Beam could be used with the Normal, Ice and Wave Beam. However, neither the Normal, Ice, nor Wave Beam could be used simultaneously. This was changed in the remake, Metroid: Zero Mission, which allows Samus to stack all of her Beams. As in Super Metroid, the beams were designed to be freely combined in a variety of combinations, though in-game it is impossible to use the Plasma Beam without the Ice Beam. An unused option in Zero Mission would also allow her to enable and disable certain Beams as in Super Metroid. 
In Metroid II, none of the beams could be stacked together. In the remake, Metroid: Samus Returns, Wave, Spazer, and Plasma Beams stack onto the Power Beam. However, the Ice Beam and Grapple Beam do not, instead functioning as separate beam options. When the Beam Burst is activated it will allow Samus to unleash a rapid fire beam with the properties or her current stacked beams, while the Ice Beam option disabled when the Beam Burst is active, thus Samus is only able to switch to the Grapple Beam when the Beam Burst is activated.
As stated, Super Metroid implemented stacking on a higher level. Most of the beams could be used simultaneously and unlike most iterations could be stacked in a wide variety of combinations, a feature only seen in Super and Zero Mission. However, the Spazer Beam and Plasma Beam were incompatible, due to memory limitations on the SNES cartridge. Nevertheless, there are certain glitches that would allow Samus to use both the Spazer and Plasma Beam simultaneously (see Murder Beam, Chainsaw Beam, and Spacetime Beam).
In Metroid Fusion, all beam upgrades stack and are collected in a fixed order with no way to unequip new beams. Unlike in Super, this means that it impossible to obtain and use later beams without them stacking.
Metroid Prime, Echoes and Hunters do not allow for stacking of Beams. Instead, there is a Beam Select option which allows Samus to switch between her Beams to overcome obstacles. Stacking akin to that in Fusion returned in Metroid Prime 3: Corruption, due to the introduction of Hypermode; the Plasma Beam overwrites the Power Beam when acquired, and the Nova Beam overwrites the Plasma Beam. With all Beams collected, Samus is able to retain the rapid fire rate of the Power Beam, the burning properties and welding abilities of the Plasma Beam, while also gaining the ability to penetrate Phazite when used with the X-Ray Visor.
While Echoes does not feature stacking in the traditional sense, the Annihilator Beam can be considered a pseudo-stack, in that it requires both the Light and Dark Beam to be used as it consumes both of their ammo types and has a heavy rate of fire on par with that of the power beam. It is capable of opening portals of either polarity, seriously harming creatures of both Aether and Dark Aether, and energizing Light Beacons and Light Crystals. However, it is incapable of opening white or purple hatches.
Like Metroid and Metroid II: Return of Samus, Metroid Prime and Metroid Prime 2: Echoes are the first two of the Metroid Prime trilogy without stacking, and like Super Metroid, Metroid Prime 3: Corruption is the first of the trilogy to introduce stacking beams.
==Power Suit upgrades==
Samus can use the Varia Suit and Gravity Suit together in Super Metroid, allowing her to resist heat damage, reduce attack damage, and move freely under water, as well as lava. This is retained in Fusion, Zero Mission and Metroid: Other M.
In Metroid Prime, Samus is able to combine her Varia Suit and Gravity Suit, much like she did in Super Metroid. There is also a new type of suit called the Phazon Suit, which protects Samus from blue Phazon. In the end, Samus is resistant to heat damage, can move freely under water, and come in contact with Phazon without taking any damage. The fully upgraded Suit does not protect her from lava or red Phazon, however.
Echoes is the first game to make an upgrade obsolete with another upgrade. The Light Suit effectively replaces the Dark Suit further in the game, as it renders the air of Dark Aether entirely harmless. The Dark Suit had previously filtered out 80% of the air's corrosive effects, causing Samus to lose 1.2 units of energy per second when exposed as opposed to 6 with the Varia Suit. She is never able to protect herself from Phazon during Echoes, however. This forces her to avoid it completely, except during the final battle with Dark Samus.
==Missile upgrades==
Metroid Fusion is the first Metroid game that allows Samus to stack her Missiles. The Super Missile in this game is an overwrite of the normal Missile, which is then enhanced by the Ice Missile, and finally the Diffusion Missile. Her Missiles become stronger with each upgrade, but also have a slower rate of fire. Metroid Dread also stacks Missile upgrades.
In other games, Super Missiles are a separate weapon with equal or lower amounts of ammunition. In Metroid Prime and Prime 2, Super Missiles are instead a Charge Combo of the Power Beam. Corruption stacks the Ice Missile effect onto Samus's normal Missiles as well, which can be enhanced with the Seeker Missiles.
==Other stacked upgrades==
Metroid Prime 3: Corruption introduces grapple stacking. As the game goes on, Samus is able to upgrade her Grapple Beam with a lasso, swing, and the ability to drain or fill objects with energy.
==Development notes==
According to Mark Pacini, stacking was already planned for Corruption before Retro Studios saw the Wii Remote for the first time. The lesser number of buttons on it compared to a Nintendo GameCube controller necessitated simpler controls, and therefore beam switching was replaced with stacking.
==Trivia==
*In Corruption, if the Nova Beam is obtained before the Plasma Beam via hacking, it does not retain the welding properties until the Plasma Beam is acquired.
==References==
Category:Miscellaneous
Category:Items