subscripts on the left side of a number I encountered this strange notation in an online set theory course. I've emailed my instructor, but he's on vacation so I was wondering if any of you knew what it meant. Apparently it equals 8.
Edit: Here's a complete question involving the notation again--I didn't give enough context the first time
 A: My guess is that $a+_nb$ is the remainder of the division of $a+b$ by $n$.
A: I think Jose Carlos Santos has the correct answer.
If so, to go into a little more detail:
1) then $12$ is a subscript of the PLUS sign (and to the right) and not of the $10$.
2) Just like $+$ or $-$ or $\times$ or $\div$, the operation $+_{12}$ is a binary operation so that $a \ +_{12} \ b$, takes two numbers, $a$ and $b$, does... something... to them, and the result is a third number $c$.  In this case, what $+_{12}$ does is adds the number and takes the remainder.
$10 +_{12} 10 = $ the remainder of $10 + 10$ divided by $12 = $ the remainder of $20$ when divided by $12 = 8$.
This is called modular arithmetic and it's applications and usefulness should be clear for systems that are period rather than infinite.  It's very useful for discussing number theory and divisiblity of integers.
For example: the familiar "rule of three" that if you add up the digits of a number, and you add up the digits of the results of that, and you add up the digits of that, then the number is divisible by $3$ if you end up with $3,6,$ or $9$ and not otherwise; becomes easy to state and prove.  You don't have to repeat "and then add those digits, and then add those" because $+_3$ is about remainders and the result will be a single digit.  So the statement is "A number is divisible by $3$ if and only if the $+_3$ of its digits is $0$".  And the proof is: $10_3 = 9 +_3 1 = 1$ so $10 \times_3 k = k_3$.  So if if $n = d_0 + 10*d_1 + 10^2*d_2 + ..... + 10^n*d_n$ then $n_3 = d_0 +_3 + 10 \times_3 d_1 + 10^2\times_3 d_2 + ..... + 10^n\times_3 d_n = d_0 +_3 d_1 +_2 d_2 +_3 .... +_3 d_n$
