The strength of the interaction depends of a factor which is called the "coupling constant", which for quarks and gluons is usually denoted as αs. Here, the index "s" stands for "strong", since the interaction between quarks and gluons had been called the "strong interaction" for reasons that show up nicely in the plot above, as we will see in a second. The exchange of one gluon is proportional to a factor g² = 4παs - in the diagram, each on of the two vertices where the gluon and the quark get in touch contributes a factor of g, the square root of 4παs