▁is ▁equivalent ▁to ▁a ▁V igen ère ▁ci pher ▁with ▁ABC DEF G HI J K LM N OP Q R ST U V W XY Z ▁as ▁the ▁key . ▁It ▁is ▁also ▁equivalent ▁to ▁a ▁Ca esar ▁ci pher ▁in ▁which ▁the ▁shift ▁is ▁increased ▁by ▁ 1 ▁with ▁each ▁letter , ▁starting ▁at ▁ 0 . ▁ ▁Us age ▁ ▁Within ▁the ▁body ▁of ▁the ▁tab ula ▁rect a , ▁each ▁alphabet ▁is ▁shift ed ▁one ▁letter ▁to ▁the ▁left ▁from ▁the ▁one ▁above ▁it . ▁This ▁forms ▁ 2 6 ▁rows ▁of ▁shift ed ▁al ph ab ets , ▁ending ▁with ▁an ▁alphabet ▁starting ▁with ▁Z ▁( as ▁shown ▁in ▁image ). ▁Se par ate ▁from ▁these ▁ 2 6 ▁al ph ab ets ▁are ▁a ▁header ▁row ▁at ▁the ▁top ▁and ▁a ▁header ▁column ▁on ▁the ▁left , ▁each ▁containing ▁the ▁letters ▁of ▁the ▁alphabet ▁in ▁A - Z ▁order . ▁ ▁The ▁tab ula ▁rect a ▁can ▁be ▁used ▁in ▁several ▁equivalent ▁ways ▁to ▁encrypt ▁and ▁dec rypt ▁text . ▁Most ▁commonly , ▁the ▁left - side ▁header ▁column ▁is ▁used ▁for ▁the ▁plain text ▁letters , ▁both ▁with ▁encryption ▁and ▁dec ryption . ▁That ▁usage ▁will ▁be ▁described ▁here in . ▁In ▁order ▁to ▁dec rypt ▁a ▁Tr ith em ius ▁ci pher , ▁one ▁first ▁loc ates ▁in ▁the ▁tab ula ▁rect a ▁the ▁letters ▁to ▁dec rypt : ▁first ▁letter ▁in ▁the ▁first ▁interior ▁column , ▁second ▁letter ▁in ▁the ▁second ▁column , ▁etc