What is a small square placed at the bottom of perpendicular lines?

Answer 1

It is an indicator that the angle formed at that point is a right angle (90 degrees).

The small square at the crossing point of two perpendicular lines serves to indicate that the two lines indeed form an angle of 90 degrees.

Answer 2

It is an indicator that the angle formed at that point is a right angle (90 degrees).

Answer 3

The small square at the crossing point of two perpendicular lines serves to indicate that the two lines indeed form an angle of 90 degrees.