Polarised light micrograph of a thin section of common brass showing the distinct grain structure of this metal alloy. The grain boundaries - the three-dimensional surfaces that enclose grains - show up as fine lines around the more or less polygonal grains. The straight, parallel sets of lines running across the polygons are twins, which show different colours to the rest of the grain because of the use of polarised light. Twinned crystals consist of two or more portions joined together so that one part is the reflection of the other. Magnification: x135 at 35mm size. Microcosmos page 128 Fig. 7.7