Take the day of the month on which the person was born and reduce it. A birth on the 7th gives 7; on the 19th, 1 plus 9 gives 10 and then 1; on the 22nd, the master number 22 is usually kept.
Meaning
The narrowest of the core numbers and the most concrete: a specific aptitude carried into the life, something the person can do rather than something they are. Read with the life path it is the tool the path is walked with. It is also the number that most often explains a talent that the rest of the chart does not account for.