character repeated in all columns, such as a line of dashes ("----...---") To avoid this problem, some printers use a staggered arrangement, with the characters in each column rotated around the drum by a different amount. Then simultaneous firing occurs only if the printed line matches the character layout on the drum, which should not happen in normal text. Lower-cost printers do not use a hammer for each column. Instead, a hammer is provided for every other column and the entire hammer bank is arranged to shift left and right, driven by an additional voice coil. For this style of printer, two complete revolutions of the character drum ara required with one revolution being used to print all the "odd" columns and another revolution being used to print all of the "even" columns; however, on the plus side, only half (plus one) the number of hammers, magnets, and the associated channels of drive electronics are required. At least one low-cost printer, made by CDC, achieves the same end by moving the paper laterally while keeping the hammer bank at rest. Dataproducts was a typical vendor of drum printers, often selling similar models with both a full set of hammers (and delivering, for example 600 lines-per-minute of output) and a half set of hammers (delivering 300 LPM). Printers with horizontally moving print elements Chain printer Chain printers place the type on a horizontally-moving circular chain. As with the drum printer, as the correct character passes by each column, a hammer is fired from behind the paper. Compared to drum printers, chain printers have the advantage that the type chain can usually be changed by the operator. A further advantage is that vertical registration of characters in a line is much improved over drum printers, which need extremely precise hammer timing to achieve a reasonably straight line of print. By selecting chains that have a smaller character set (for example, just numbers and a few punctuation marks), the printer can print much faster than if the chain contains the entire upper- and lower-case alphabet, numbers, and all special symbols. This is because, with many more instances of the numbers appearing in the chain, the time spent waiting for the correct character to "pass by" is greatly reduced. Common letters and symbols appear more often on the chain, according to the frequency analysis of the likely input. It is also possible to play