bottom and are described as "X over Y over Z." Dwarf signals are smaller signals used in low speed or restricted clearance areas. Most signaling aspect systems have a parallel set of aspects for use with dwarf signals that differ from aspects used in high signals. Dwarf signals may have multiple heads just like a high signal, but sometimes dwarf signals use so-called "virtual heads" to save on space and cost. This is where a dwarf signal displays multiple lamps on what would ordinarily be a single signal head creating the effect of multiple signal heads. For example, a stack of dwarf lamps in the order Yellow/Red/Green can display plain Yellow, Red and Green as well as Yellow over Green and Red over Green. Behind the signal head is placed a dark backing or target, which helps improve signal visibility in bright ambient lighting. Target designs vary, but are usually round or oval, depending on the layout of the signal lamps. For each type of signal there are usually a range of target dimensions that can be chosen by the individual railroad company. As dwarf signals are not designed to be seen from long distances, they are not generally equipped with targets. Mounting Signals are most commonly mounted on trackside masts about to high to put them in the eyeline of the engineer. Signals can also be mounted on signal bridges or cantilever masts spanning multiple tracks. Signal bridges and masts typically provide at least of clearance over the top of the rail. Bracket masts are arranged with multiple signals are mounted on the same masts governing two adjacent tracks. Bracket masts tend to be the tallest type of signal to allow the train crew to see the signal over a train on the intervening track. Signals in electrified territory may be mounted on the catenary structure, and signals on bi-directional lines may be mounted back-to-back on the same mounting device. Prior to 1985, signals were required by regulation to be mounted above and to the right of the track they governed. This mounting was designed to allow the engineer to view the signal when driving a steam or diesel locomotive with a long hood that restricted the view to the left. In most situations, especially where bi-directional running was implemented, signals needed to be mounted above the track or on bracket masts to allow this right hand placement. As