how does a plane mirror help to take proper readings in meters without parallax error?where are the mirrors placed?

A parallax errors means if we do not align our eye so that it is perpendicular to the pointer made on the scale. If this occurs, the pointer seems to line up with the wrong mark on the scale and thus parallax error takes place while taking readings. This error can be minimized by fitting a plane mirror behind the pointer in such as way that the mirror lies in the same plane as the scale lies. The plane mirror helps the observer in finding the correct point of view by the reflection of the pointer which is hidden behind the pointer.
Thus, a plane mirror is placed just below the pointer of a scale. You might have seen the plane mirror just below the scale in electrical instruments like in voltmeter, ammeter etc.