The direction of the front of the model's base is assumed to be its facing, the actual direction faced by the creature it represents. A model cannot charge or shoot at something it cannot see. To represent this, a target must lie within a 90° arc projected from the front of the model. Where models are on square bases, this can be imagined by projecting a line through the corners. This is the 'arc of sight' and determines which opponents the model can shoot at or charge (see Diagram 8.1).
Source: Warhammer Fantasy: 7th Edition
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