It's usually easiest to use the 3D Cursor to mirror bones. - Position 3D Cursor at the center (Shift+S -> Cursor to Center) - Switch Pivot Point to 3D Cursor (Period Key, or the drop down on the header) - Select the bones that need to be symmetrical - Duplicate (Shift+D) and Cancel (Esc) so bones duplicate but aren't moved - Mirror (Ctrl+M -> choose appropriate constraint: X, Y, or Z, click to confirm) it should use the 3D Cursor as the pivot point. If you name things using .L or .R you can use the Flip Names option in the Armature Menu. It'll change Arm.L to Arm.R for you, but only works with that exact naming syntax.