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, despite its messages being shorter, [[Morse code]] is harder to send by tapping or banging,. becauseIts Morseshort codeand islong timesignals basedcan onbe signalimprovised presentas taps and thumps, or notshort present.and long Morsewhistles codeor alsoscraping takessounds, longerbut tap codes are simpler to learn,whereand can be used in a wider variety of situations.<ref name="denton">{{cite book |last1=Denton |first1=Jeremiah A. |last2=Brandt |first2=Ed |title=[[When Hell Was in Session]] |date=1976 |publisher=Readers Digest Press |___location=New York |isbn=0-88349-112-5}}</ref> theThe 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.
