There are several different stances you can unlock in Halo 4, though I’m fairly certain most of the kids online will be using the Assassin stance (am I right?).
Here are the different stances you can unlock, and how to unlock them.
Assassin – Unlocked at rank SR-32
Assault – Unlocked with the commendation First Strike – Master
Believe – Unlocked with the commendation Forerunner Ordinance – Master
Breach – Unlocked with the commendation UNSC Weapons – Master
Heroic – Unlocked at rank SR-10
Last Stand – Unlocked at rank SR-50
Recruit – Unlocked by default
Standoff – Unlocked with the commendation Ordnance – Master
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