Experiment with the prompts after you select "arc" from the PLINE prompt:
Current line-width is 0.0000
Arc/Close/Halfwidth/Length/Undo/Width/<Endpoint of line>:
Arc/Close/Halfwidth/Length/Undo/Width/<Endpoint of line>: a
<Endpoint of arc>: a
Included angle: 90
CE specifies the center (and therefore direction),
D specifies the direction,
A specifies the angle, and then the mouse determines the direction,
S specifies the second point on the arc, which, after the third is picked,
gives the direction too.
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