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Specifically: Expand the history section, particularly Street Fighter Alpha.
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Various types of projectile attacks exist in the Street Fighter series, and allow users to damage foes from a distance. Depending on the projectile itself, it can also serve many other purposes, such as controlling space and pressuring opponents. However, projectiles can be jumped over somewhat easily.
HistoryRyu and his Hadoken would go on to become a two of the series' many icons. Sagat, the final boss, was the only other fighter with a projectile, not counting Ken, who at the time was a multiplayer-exclusive palette swap of Ryu. Both Ken and Sagat would become playable in single-player modes from Street Fighter II on.
With the introduction of more characters into the series, several more projectile attacks and users appeared, and opened up more varied combo tactics as a result.
The Hadoken played a major part in the divergence - both in the story and in gameplay - of Ryu and Ken. Ryu was shown to have gained more proficiency in the 'Hado' principle of his style, and thus was able to use more variants of the technique, including the Shakunetsu Hadoken and the Shinku Hadoken, his first and "trademark" Super Combo.
The mechanics for projectiles in the Street Fighter Alpha games made them much weaker overall, though they were still quite useful in combos.
With the introduction of the parry mechanic, all users (particularly non-projectile users) gained an advantage against projectiles, being able to nullify them completely, as opposed to jumping, or blocking (and thus taking Chip Damage).
In addition, from this series on, the EX Special versions of most projectile attacks hit twice, sometimes via firing two projectiles at once.
In Street Fighter X Tekken, one of the exceptions to the above, many Tekken characters are given moves that allow them to "sidestep" and avoid projectiles; this is due in part to the Tekken series' nature as a 3D fighter, and the relative lack of projectile users (and, therefore, lack of a proper defense besides blocking).