The target point of the beam is steered around the front of the tube though the use of deflection magnets or electrostatic plates. Storage tubes were based on CRTs, sometimes unmodified. They relied on two normally undesirable principles of phosphor used in the tubes. One was that when electrons from the CRT's electron gun struck the phosphor to light it, some of the electrons "stuck" to the tube and caused a localized static electric charge to build up. The second was that the phosphor, like many materials, also released new electrons when struck by an electron beam, a process known as secondary emission. Secondary emission had the useful feature that the rate of electron release was significantly non-linear. When a voltage was applied that crossed a certain threshold, the rate of emission increased dramatically. This caused the lit spot to rapidly decay, which also caused any stuck electrons to be released as well. Visual systems used this process to erase the display, causing any stored pattern to rapidly fade. For computer uses it was the rapid release of the stuck charge that allowed it to be used for storage. In the Williams tube, the electron gun at the back of an otherwise typical CRT is used to deposit a series of small patterns representing a 1 or 0 on the phosphor in a grid representing memory locations. To read the display, the beam scanned the tube again, this time set to a voltage very close to that of the secondary emission threshold. The patterns were selected to bias the tube very slightly positive or negative. When the stored static electricity was added to the voltage of the beam, the total voltage either crossed the secondary emission threshold or didn't. If it crossed the threshold, a burst of electrons was released as the dot decayed. This burst was read capacitively on a metal plate placed just in front of the display side of the tube. There were four general classes of storage tubes; the "surface redistribution type" represented by the Williams tube, the "barrier grid" system, which was unsuccessfully commercialized by RCA as the Radechon tube, the "sticking potential" type which was not used commercially, and the "holding beam" concept, of which the Selectron is a specific example. In the most basic implementation, the holding beam tube uses three electron guns; one for writing, one for reading, and a third "holding gun" that maintains the pattern. The general operation is very similar to the Williams tube in concept. The main difference was the holding gun, which fired continually and unfocussed so it covered the entire storage area on the phosphor.