4th Kyū
Guide
With a physical chess board in front of you, experienced chess players can determine legal moves for all pieces almost unconsciously.
However, in blindfold chess, you must determine legal moves based solely on text-based information like 'a knight is placed on c3'. This is surprisingly difficult if you're not used to it.
Let's start by considering the relatively simple case of determining the King's legal moves. The King can move in all directions, but only one square at a time.
For any piece, the concept of change (delta) is important.
For example, let's think about rank change. When moving from g1 to h2, the rank difference is 1. For a8 to a6, the difference is 2. This is what mathematicians call absolute value, but we'll use the friendlier term 'change' here.
File change works the same way — assign numbers to files (a=1, b=2, ... h=8) and calculate the difference.
The King can move when the maximum of the rank change and file change is 1, and both are not 0. Expressed as a formula: