Sodasi-Vidya(16 letter mantra)
The Sodasi  is substantially the same as pancadasi using same letters in the same order, but adding the seed-syllable like SRIM or AIM or KLIM at the end which is regarded as “secret”


           
OM  KA  E  I  LA  HRIM  HA  SA  KA  HA  LA  HRIM  SA  KA  LA  HRIM  SRIM


or      
     

OM  KA  E  I  LA  HRIM  HA  SA  KA  HA  LA  HRIM  SA  KA  LA  HRIM  AIM


or
      

OM  KA  E  I  LA  HRIM  HA  SA  KA  HA  LA  HRIM  SA  KA  LA  HRIM  KLIM


There are also extensions of panchadasi mantra  like