In a keyboard operation two sequential keystrokes are used for generation of a character code. A rule is set up according to which some preset combinations of two sequential keystrokes are valid and others are not. The rule is implemented by storing the validity conditions into a table in a control unit. According to a preferred rule the second key to be operated must be located in a column to the right of the column where the first key is located, otherwise an error signal will be generated. The method is preferably used for a one hand keyboard with three times four keys.