Transformers perform the simple but vital task of increasing or decreasing AC voltage by means of electromagnetic flux. Many specialised types exist but in many cases a suitable transformer can be chosen from one of the many universal types. Most of these are based around laminated steel cores with the 'E/I' configuration being one of the most popular. Many transformers of this type are chassis mountable, but smaller PCB transformers are also available and toroidal transformers also offer some advantages, such as smaller size and reduced hum, often making them preferable in some audio applications. Choosing the right transformer should present few problems, with the main considerations being power, input voltage and output voltage. Some transformers are equipped with two sets of secondary windings, typically offering two identical sets of output terminals, allowing for some degree of flexibility of use. Some may offer multiple winding take-off points, thereby giving a range of different secondary voltages according to which terminal is selected. Those looking for infinitely variable output voltages may wish to consider a variable transformer, which is usually of toroidal form but featuring a sliding contact to effectively lengthen or shorten the secondary windings as required. Another important consideration not to be overlooked is a transformer's physical size.