»Ys

While looking up the mythical town in France, Wikipedia's helpful disambiguation page pointed me in the direction of the yoctosecond, a unit of time representing one-quadrillionth of a second. Yocto is the smallest of the SI units, denoting a factor of 10^−24. The yotta in yottasecond, with the same ys abbreviation, denotes a factor of 10²⁴ and is the largest SI unit.