12×9= 108

23×9=207

34×9=306

Ten digits are less than single digits 1. Two digits multiplied by 9 equals one digit multiplied by 9, and then a 0 is added in the middle.

You see, 4×9=36, with 0 34× 9 = 306 in the middle. This is just one kind, and there are many summaries.