A mounting arrangement for a light mountable on a mounting rail comprising: a light body containing a light source, a source of electrical energy and a switch for selectively coupling the light source and the source of electrical energy in circuit for producing light; a mounting arrangement on said light body for mounting said light body on a mounting rail having a keying feature, said mounting arrangement comprising: first and second members (52, 54) movable closer together and farther apart, said light body having a recess between said first and second members (52, 54); a keying member removably disposed in the recess of said light body, said keying member having a keying feature that engages the keying feature of the mounting rail characterised in that the first and second members (52, 54) are clamp members movable closer together for clamping the mounting rail and farther apart for releasing the mounting rail, said mounting arrangement including a clamp screw for moving the first and second clamp members (52, 54) closer together and farther apart; the keying member having a key body removably disposed in the recess of said light body between the first and second clamp members (52, 54) in a predetermined location with respect to a fixed one of said clamp members, said keying member being separate from said clamp screw, wherein said light is mountable on mounting rails having keying features of different sizes and shapes, and wherein said keying member is selected from a set of keying members for having a keying feature of a size and shape that corresponds to the size and shape of the keying feature of the mounting rail. a light body containing a light source, a source of electrical energy and a switch for selectively coupling the light source and the source of electrical energy in circuit for producing light; a mounting arrangement on said light body for mounting said light body on a mounting rail having a keying feature, said mounting arrangement comprising: first and second members (52, 54) movable closer together and farther apart, said light body having a recess between said first and second members (52, 54); a keying member removably disposed in the recess of said light body, said keying member having a keying feature that engages the keying feature of the mounting rail characterised in that the first and second members (52, 54) are clamp members movable closer together for clamping the mounting rail and farther apart for releasing the mounting rail, said mounting arrangement including a clamp screw for moving the first and second clamp members (52, 54) closer together and farther apart; the keying member having a key body removably disposed in the recess of said light body between the first and second clamp members (52, 54) in a predetermined location with respect to a fixed one of said clamp members, said keying member being separate from said clamp screw, wherein said light is mountable on mounting rails having keying features of different sizes and shapes, and wherein said keying member is selected from a set of keying members for having a keying feature of a size and shape that corresponds to the size and shape of the keying feature of the mounting rail. first and second members (52, 54) movable closer together and farther apart, said light body having a recess between said first and second members (52, 54); a keying member removably disposed in the recess of said light body, said keying member having a keying feature that engages the keying feature of the mounting rail characterised in that the first and second members (52, 54) are clamp members movable closer together for clamping the mounting rail and farther apart for releasing the mounting rail, said mounting arrangement including a clamp screw for moving the first and second clamp members (52, 54) closer together and farther apart; the keying member having a key body removably disposed in the recess of said light body between the first and second clamp members (52, 54) in a predetermined location with respect to a fixed one of said clamp members, said keying member being separate from said clamp screw, wherein said light is mountable on mounting rails having keying features of different sizes and shapes, and wherein said keying member is selected from a set of keying members for having a keying feature of a size and shape that corresponds to the size and shape of the keying feature of the mounting rail.