Here's a picture of the whole thing.
So you've lost the chain slack gauge, or didn't get one with your bike.
You can make one!
Here's a picture of the whole thing.
and here's a closeup of the business end.
Peter Paulovits in Germany reminded me that most of the world is metric, and supplied the appropriate numbers:
| Metric | English |
|---|---|
| A = 33 mm | 1.3 inches or 1-5/16 " |
| B = 43 mm | 1.7 inches or 1-11/16 " |
| C = 53 mm | 2.1 inches or 2-2/32 " |
And here's how to use it.