Why is a signed byte's lowest value -128?

Question

Howdy, I googled online and searched on here for the answer but no dice, I do apologize if this has been asked before or that I didn't use the proper search terms.
I understand why the largest value is 127, but I'd assumed that the lowest value would be -127.
01111111 is 127
11111111 is -128?
I believe that the 8th bit is to indicate weather the value is a positive or a negative, and the maximum value for 7 bits would be 127. So why is it 128 when it's negative?

Answer 1

https://www.sololearn.com/Discuss/2090896/?ref=app

Answer 2

Avinesh
If
0 | 111 = 7
Why is
1 | 111 = -8?
The only difference I can see is the 4th bit making it negative. 3 bits can only represent up to 7. Can you explain in layman's terms why it's -8? And yes i did read the thread.

Answer 3

Oh, so it's using two's compliment. I was looking at it as the signed magnitude representation (SMR). Thanks for clearing that up!