198

This number is a composite.

198 is the first number between a twin prime pair such that if multiplied by its reversal (198 * 891) does the same! [Luhn]

198 can be formed by concatenating 19 and 8. Note that 19 is the 8th prime number. Curiously, 198 is the (19*8)nd composite number! [Fazely]

The difference between any odd-digit emirp pairs is divisible by 198. [Green]