The nine horizontal and nine vertical lines on an $8\times8$ checkerboard form $r$ rectangles, of which $s$ are squares.  The number $s/r$ can be written in the form $m/n,$ where $m$ and $n$ are relatively prime positive integers.  Find $m+n.$
1. **Counting the Rectangles:**
   - To count the total number of rectangles in an $8 \times 8$ checkerboard, we need to choose 2 out of the 9 horizontal lines and 2 out of the 9 vertical lines. 
   - The number of ways to choose 2 lines from 9 is given by the binomial coefficient $\binom{9}{2}$.
   - Therefore, the total number of rectangles is:
     \[
     r = \binom{9}{2} \times \binom{9}{2} = \left(\frac{9 \times 8}{2}\right)^2 = 36^2 = 1296
     \]

2. **Counting the Squares:**
   - To count the number of squares, we need to consider squares of different sizes.
   - A square of side length $k$ can be placed in $(9-k) \times (9-k)$ positions on the checkerboard.
   - Therefore, the number of $k \times k$ squares is $(9-k)^2$.
   - Summing over all possible side lengths from 1 to 8, we get:
     \[
     s = \sum_{k=1}^{8} (9-k)^2 = 8^2 + 7^2 + 6^2 + 5^2 + 4^2 + 3^2 + 2^2 + 1^2
     \]
   - Calculating the sum:
     \[
     s = 64 + 49 + 36 + 25 + 16 + 9 + 4 + 1 = 204
     \]

3. **Finding the Ratio \( \frac{s}{r} \):**
   - We have \( s = 204 \) and \( r = 1296 \).
   - The ratio is:
     \[
     \frac{s}{r} = \frac{204}{1296}
     \]
   - Simplifying the fraction by dividing both the numerator and the denominator by their greatest common divisor (GCD), which is 12:
     \[
     \frac{204 \div 12}{1296 \div 12} = \frac{17}{108}
     \]

4. **Expressing \( \frac{17}{108} \) in the form \( \frac{m}{n} \):**
   - Here, \( m = 17 \) and \( n = 108 \).
   - Since 17 and 108 are relatively prime, the fraction is already in its simplest form.

5. **Finding \( m + n \):**
   - We have \( m = 17 \) and \( n = 108 \).
   - Therefore, \( m + n = 17 + 108 = 125 \).

The final answer is \( \boxed{125} \)