Rules of Movement

The King can move only one square at a time, vertically, horizontally, or diagonally. While this is obvious on a physical board, in blindfold chess, calculating the legal moves by using the origin and destination squares can provide certainty.

A King can move from a square $(file_1, rank_1)$ to another square $(file_2, rank_2)$ only if the following condition is met:

This means that if the maximum of the absolute differences in file and rank is 1, it is a legal move. The following expressions represent all 8 possible moves for the King:

• Horizontal Movement: $(1, 0)$
• Vertical Movement: $(0, 1)$
• Diagonal Movement: $(1, 1)$

Since mathematical explanations alone can be difficult to understand, let's look at some practical examples.

Examples

e4 → e5 (Vertical)

1. Start Position: e4 = $(5, 4)$
2. Target Position: e5 = $(5, 5)$
3. Calculation:
   • File difference: $|5 - 5| = 0$
   • Rank difference: $|4 - 5| = 1$
   • Maximum value: $max(0, 1) = 1$
   • Result: ✅ Legal Move

d3 → e4 (Diagonal)

1. Start Position: d3 = $(4, 3)$
2. Target Position: e4 = $(5, 4)$
3. Calculation:
   • File difference: $|4 - 5| = 1$
   • Rank difference: $|3 - 4| = 1$
   • Maximum value: $max(1, 1) = 1$
   • Result: ✅ Legal Move

c2 → e4 (Invalid)

1. Start Position: c2 = $(3, 2)$
2. Target Position: e4 = $(5, 4)$
3. Calculation:
   • File difference: $|3 - 5| = 2$
   • Rank difference: $|2 - 4| = 2$
   • Maximum value: $max(2, 2) = 2$
   • Result: ❌ Illegal Move