System springs, they have rectangle cross sections.
Produced according to ISO 10243 which defines its diameter,
length and strength. This standard defines four series of springs,
which when of same size they are interchangeable offering different
forces. These four series are each one colour for an easy
identification of the type of spring.
Inside the family of rectangle cross section springs there is a
fifth serie which is superstrong, which with equivalent sizes to
the other four series it offers a force up to four times the one of
their strongest serie.
Also there is another type of system spring which follows the
same rules as the previous, but this type has an oval cross
section which keeping the same sizes it offers alternative
forces and strokes.