Any substitution cipher, enciphered by a single alphabet composed of letters, figures or conventional signs, can be handled by the methods of case 6. For example, the messages under case 4-a and 5-a are easily solved by these methods. But note that the messages under case 4-b and 5-b cannot so be solved because several alphabets are used. We will see later that there are methods of segregating the different alphabets in some cases where several are used and then each of the alphabets is to be handled as below. Case 6-a. Message QDBYP BXHYS OXPCP YSHCS EDRBS ZPTPB BSCSB PSHSZ AJHCD OSEXV HPODA PBPSZ BSVXY XSHCD This message was received from a source which makes us sure it is in Spanish. The occurrence of B, H, P and S has tempted us to try the first two words as in case 4 and 5 but without result. We now prepare a frequency table, noting at the same time the preceding and following letter. This latter proceeding takes little longer than the preparation of an ordinary frequency table and gives most valuable information. Frequency Table Prefix Suffix A 11 2 ZD JP B 11111111 8 DPRPBSPZ YXSBSPPS C 11111 5 PHSHH PSSDD D 11111 5 QECOC BROA E 11 2 SS DX F G H 111111 6 XSSJVS YCSCPC I J 1 1 A H L M N O 111 3 SDP XSD P 111111111 9 YXCZTBHAB BCYTBSOBS Q 1 1 D R 1 1 D B S 111111111111 12 YYCBBCPHOPBX OHEZCBHZEZVH T 1 1 P P U V 11 2 XS HX X 11111 5 BOEVY HPVYS Y 1111 4 BHPX PSSX Z 111 3 SSS PAB It is clear from an examination of this table that we have to deal with a single alphabet but one in which the letters do not occur in their regular order. We may assume that P and S are probably A and E, both on account of the frequency with which they occur and the variety of their prefixes and suffixes. If this is so, then B and H, are probably consonants and may represent R and N respectively. D and X are then vowels by the same method of analysis. Noting that HC occurs three times and taking H as N we conclude that C is probably T. Substitute these values in the last three words of the message because the letters assumed occur rather frequently there. PBPSZBSVXYXSHCD