Points $A,B,C,D$ have been marked on checkered paper (see fig.). Find the tangent of the angle $ABD$. 
[img]https://cdn.artofproblemsolving.com/attachments/6/1/eeb98ccdee801361f9f66b8f6b2da4714e659f.png[/img]
1. **Identify the coordinates of the points:**
   - Let's assume the coordinates of the points on the checkered paper.
   - Suppose \( A = (0, 0) \), \( B = (1, 0) \), \( C = (1, 1) \), and \( D = (3, 1) \).

2. **Verify the right angle at \( B \):**
   - The triangle \( ABC \) is right-angled at \( B \) because \( AB \) is horizontal and \( BC \) is vertical.
   - Calculate the lengths:
     \[
     AB = \sqrt{(1-0)^2 + (0-0)^2} = 1
     \]
     \[
     BC = \sqrt{(1-1)^2 + (1-0)^2} = 1
     \]
   - Since \( AB = BC \), triangle \( ABC \) is an isosceles right triangle.

3. **Determine the ratio \( \frac{AD}{DC} \):**
   - Calculate the lengths:
     \[
     AD = \sqrt{(3-0)^2 + (1-0)^2} = \sqrt{9 + 1} = \sqrt{10}
     \]
     \[
     DC = \sqrt{(3-1)^2 + (1-1)^2} = \sqrt{4} = 2
     \]
   - Therefore, the ratio is:
     \[
     \frac{AD}{DC} = \frac{\sqrt{10}}{2}
     \]

4. **Find the tangent of \( \angle ABD \):**
   - Let \( \angle ABD = \alpha \).
   - Using the tangent definition:
     \[
     \tan \alpha = \frac{\text{opposite}}{\text{adjacent}} = \frac{AD}{BD}
     \]
   - Since \( BD = DC \) (as \( B \) and \( D \) are horizontally aligned):
     \[
     BD = 2
     \]
   - Therefore:
     \[
     \tan \alpha = \frac{\sqrt{10}}{2}
     \]

The final answer is \(\boxed{\frac{\sqrt{10}}{2}}\).