First of all PAN Number is a 10 digit unique number which 
comprises of 5 Alpha digits then 4 numeric and last again 
alpha digit.

The first three characters in PAN number are unique series 
provided by the authority.

The 4th alphabet in the PAN is based on the type of 
assessee i.e. either company, individual, HUF, etc etc

The 5th digit in the PAN is the assessee last name/surname 
first character..say if its Somesh Chauhan, the 5th will 
be 'C'.