I don't fully understand this, can any of you explain please?

in CCNA & CCENT
Hello everyone! I am studying for my CCENT and I am going through the Lammle book. There is something I don't understand.
Now on managing the configuration register there is this part:
The default configuration setting on Cisco routers is 0x2102. This means that bits 13, 8, and 1 are on, as shown below:
Configuration Register
2. 15 =0/14=0/13=1/12=0/11=0
1. 10=0/9=0/8=1/7=0/6=0
0. 5=0/4=0/3=0
2. 2=0/1=1/0=0
I just don't understand how you know 2 equals 13 or how 1 equals 8 ect. There is something I am missing here, can someone explain it to me? Thanks!
Now on managing the configuration register there is this part:
The default configuration setting on Cisco routers is 0x2102. This means that bits 13, 8, and 1 are on, as shown below:
Configuration Register
2. 15 =0/14=0/13=1/12=0/11=0
1. 10=0/9=0/8=1/7=0/6=0
0. 5=0/4=0/3=0
2. 2=0/1=1/0=0
I just don't understand how you know 2 equals 13 or how 1 equals 8 ect. There is something I am missing here, can someone explain it to me? Thanks!

Comments
2 | 1 | 0 | 2
=
0010|0001|0000|0010
Count backwards starting from the first bit (0). The first, second, and third 1's (on) are 1, 8, and 13.
Hope that makes sense?
Makes total sense now, thank you so much!