Indexes of data bits in PC
Think of their binary weights. The value of each bit is \$ 2^b \$ where \$ b \$ is the bit number. Thus the least significant bit is worth \$ 2^0 = 1 \$ and the MSB is worth \$ 2^{n-1} \$.


\$ 2^0 = 1 \$
\$ 2^1 = 2 \$
\$ 2^2 = 4 \$
etc.


You can also think of if as an offset from the least significant bit. LSB has an offset of 0 and MSB has an offset of n - 1.



Here's a 4-bit binary number sequence with decimal representations:

Bit #  3 2 1 0  |
Weight 8 4 2 1  | Decimal
----------------+--------
       0 0 0 0  |  0
       0 0 0 1  |  1
       0 0 1 0  |  2
       0 0 1 1  |  3
       0 1 0 0  |  4
       0 1 0 1  |  5
       ....
       1 1 1 0  | 14
       1 1 1 1  | 15


Look at binary value for decimal 14, '1110'. To convert to decimal we just add the bits that are on, bits 3, 2 and 1 as follows: \$ 1110_2 = 2^3 + 2^2 + 2^1 = 8 + 4 + 2 = 14_{10}\$.

This is the logical way to do this. If we used a system starting at 1 then the converstion would be \$ 1110_2 = 2^{4-1} + 2^{3-1} + 2^{2-1} = 8 + 4 + 2 = 14_{10}\$. This would be a pain.



Decimal notation

In our decimal notation we do the same thing. A three-digit number, say 987, is made up of 100s, 10s and units. It's value is \$ 9 \times 10^2 + 8 \times 10^1 + 7 \times 10^0 \$. Again, the least significant digit has an index of 0.