The square is one of the simple solving techniques that works within a grid. Finding them requires a keen eye!

The principle

Some false sign configurations can leave empty boxes on rows, boxes which must contain a true sign  :

Look at the yellow boxes forming a square: there must be at least one true sign in column b4, and another in column b5:

We note that there are boxes which are always false,  whatever the position of the true  signs : When we encounter this square configuration, we can therefore fill the boxes outside the square with false signs.

Attention

The rows do not need to be close together:

Where are the false squares? Actilud’s step-by-step solver gives us the answer:

Squares aren’t necessarily 2 x 2, even though they’re the most numerous. Here’s a 3×3 square… Of course, in most cases, the squares are well hidden. Can you find the false squares in the following grid? There are 4 left to discover…