When writing a number in scientific notation, there should be only one digit to the left of the decimal point.

Here's why:

* Conciseness: Scientific notation is designed to represent very large or very small numbers efficiently. Having only one digit before the decimal point makes the notation compact and easy to read.

* Standardization: This convention helps ensure consistency across scientific disciplines.

Example:

The number 3,450,000 in scientific notation is written as 3.45 x 10^6.