Because of the difficulty and length of time required for specifying a single letter, prisoners often devise abbreviations and acronyms for common items or phrases, such as "GN" for ''Good night'', or "GBU" for ''God bless you''.<ref name="pbs"/>
By comparison, [[Morse code]] is harder to send by tapping or banging because a single tap will fade out and thus has no discernible length. Morse code, however,it requires the ability to create two clearly distinguishable lengths (or types)forms of taps.tap, Tosuch simulateas Morsevarying bythe tapping therefore requires either two different sounds (pitch, volume), or very precise timing, so that a dash within a character (e.g. the character N, {{Morse|dash|dot|noindent=yes}}) remains distinguishable from a dot at the end of a character (e.g. E-E, {{Morse|dot|aspace|dot|noindent=yes}})volume. Morse code also takes longer to learn., Learningwhere the tap system simply requires one to know the alphabet and the short sequence "AFLQV" (the initial letter of each row), without memorising the entire grid. For example, if a person hears four knocks, they can think "A... F... L... Q". If after a pause there are three knocks, they think "Q... R... S" to arrive at the letter S.
