The fundamental element of CHEX is the square, as it is in chess. A ‘field’ consists of two squares of the same colour. Although a CHEX square may be one of two in a field it is still an entity in its own right and remains the basic unit of movement.
a1, c2 and d7 are examples of squares. f4-f5 and h2-h3 are examples of fields.
Any piece entering an empty field can make use of either square (subject to the laws governing that piece – eg, a king may not pass through or enter into CHEX).
When moving into an empty field the three options for that piece are:
1) occupy the first square
2) occupy the adjacent second square
3) if permissable, move onto the next available square and leave the field.
If option 3 is taken, that field is considered as one unit of movement (important when understanding how pawns operate).