X (dek) is a transdecimal digit. It equals ten. "Dek" can be double five ().
- An even number.
- A composite number.
- A triangular number.
- A tetrahedral number.
- Smallest nontrivial (i.e. neither 0 nor 1) number which is both triangular and tetrahedral, the other numbers are X0, X84, 4170.
- A semiprime.
- A squarefree and cubefree number.
- The symbol is an stylised X as X is a roman numeral of 10.
- That's why Dwiggin's made that symbol of 10.