The property of a mirror to flip an image about a vertical axis is called lateral inversion. Mirrors, in fact lie - the image we see of ourselves in a mirror is not how others see us. We see a laterally inverted image, other people see the "real" image. The letters of "Ambulance" are written flipped over from left to right so that this very property of mirrors can be exploited - seeing an approaching ambulance in a rear view mirror will be easy because the letters read correctly when seen in a mirror!
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