Straddling Checkerboard (Monome–Dinome)

What it is

• A variable-length numeric substitution. Ten columns labeled 0–9; the top row holds the most common letters as single digits. Two row digits (R1, R2) mark the start of two-digit codes; letters in those rows use RkC (two digits). Often I/J are combined.
• Hand rule: read one digit; if it’s R1 or R2, read a second digit to form a two-digit code; otherwise it’s a single-digit top-row letter.

Key facts you need

• You must know (or rather, infer) the mixed alphabet and which digits are R1, R2.
• Top row uses single digits excluding the two row digits.
• Rows R1 and R2 each use two-digit codes of the form RkC (k∈{1,2}).

Building the board (by keyword)

1. Choose a mixed alphabet: write unique letters from a keyword, then remaining A–Z (optionally combine I/J).
2. Pick R1 and R2 (two distinct digits 0–9).
3. Draw a 3×10 board with columns 0..9. In the top row, leave the cells at R1 and R2 blank. Fill the other 8 cells left→right with the first 8 letters of the mixed alphabet (the high-frequency set you want as single digits).
4. Continue filling remaining letters into row R1 (cells 0..9), then row R2, left→right.
   Result: 8 single-digit letters in top row; the rest are two-digit codes starting with R1 or R2.

Example board (I/J combined)

• Mixed alphabet from KEYWORD: KEYWORDABCFGHI/JKLMPQSTUVXYZ
• Choose R1=3, R2=7
• Columns: 0 1 2 3 4 5 6 7 8 9

Top row (single digits; blanks at 3 and 7):

0 1 2   4 5 6   8 9
K E Y   W O R   D A

Rows R1=3 and R2=7 (two-digit codes):

Row 3: B C F G H I L M P Q
Row 7: S T U V X Y Z (and remaining letters)

Interpretation

• Single digits: 0→K, 1→E, 2→Y, 4→W, 5→O, 6→R, 8→D, 9→A.
• Two digits: 3x → row 3, column x; 7x → row 7, column x.

Decoding example

Cipher digits: 1 5 3 4 3 5 9 7 0 7 2

Step-by-step

• 1 → top row col 1 → E
• 5 → top row col 5 → O
• 3 → row marker R1 → read next digit 4 → row 3, col 4 → H
• 3 → R1 again → read next digit 5 → row 3, col 5 → I (I/J combined)
• 9 → top row col 9 → A
• 7 → row marker R2 → next digit 0 → row 7, col 0 → S
• 7 → R2 again → next digit 2 → row 7, col 2 → U
  Plaintext: EOHIASU (toy example—actual board composition will determine exact letters).

Solving without the board

• Identify R1 and R2: digits that frequently occur and are followed by a second digit are likely row markers.
• Segment the stream: treat R1x and R2x as two-digit codes; others as single digits.
• Frequency: single-digit letters are more common (top row often holds ETAOINSH).
• Cribs: try placing THE/AND by mapping their digit patterns; adjust the board accordingly.
• Reconstruct the mixed alphabet by consistent placement as you identify letters.

Common mistakes

• Treating R1 or R2 as single letters (they are prefixes).
• Forgetting to skip R1 and R2 positions when filling top row.
• Mismatching I/J policy between encoding and decoding.

Quick reference

• Read a digit; if it is R1 or R2, read one more to form RkC.
• Top row: single-digit letters at all columns except the two row-digit columns.
• Rows R1 and R2: two-digit codes RkC.

Practice

1. With R1=3, R2=7 and top row K E Y _ W O R _ D A, decode: 1 5 3 4 3 5 9 7 0 7 2.
2. Identify likely row digits and segment: 7 1 3 2 5 7 8 0 7 9.

Answers

1. EOHIASU (given the example board above; your exact board may differ).
2. Likely R digits: 7 and 3; segmentation: 7,1,3,2,5,7,8,0,7,9 → 7x pairs at (7,8), (7,9); 3x pair at (3,2); remaining singles 1,5,0.