Hello, I am trying to learn about Mesh Current Analysis so I have a better understanding of electrical circuits.

I was reading this explanation on the subject from electronics-tutorials.ws:

http://www.electronics-tutorials.ws/dccircuits/dcp_5.html

and one of the equations is -20 = -40i1 + 60i2

I understand where -40i1 + 60i2 comes from, and I'm assuming -20 is taken from the voltage and it is given as negative because the loop goes clockwise, can someone confirm why -20 is negative, please?

Thanks