StyleSheet#hairlineWidth

This is defined as the width of a thin line on the platform. It can be used as the thickness of a border or division between two elements. Example:

This constant will always be a round number of pixels (so a line defined by it look crisp) and will try to match the standard width of a thin line on the underlying platform. However, you should not rely on it being a constant size, because on different platforms and screen densities its value may be calculated differently.