There are three types of sequences:
1) You need THREE HIGH CARDS THAT TOUCH when defending vs no trump and only TWO HIGH CARDS that touch when defending vs a suit contract
K Q J x x – lead the TOP of a 3 card or longer touching sequence (the King)
Why not lead the Jack?
By leading the King you are promising your partner you have the Queen & Jack.
If you led the Jack, Partner would not know you had the King and Queen!
If you lead the Queen, you are promising the Jack & Ten.
K Q x x
LEAD THE KING to drive out the Ace and promote a winner. By the third time the suit is played declarer may be void!BROKEN SEQUENCE
2) Q J 9 x x
Treat the same as a regular sequence and lead the Q, the top of touching.
3) A J 10 x x
Lead the top of the touching high cards (the J) when defending against no trump.
If in doubt, just remember, you always lead THE TOP OF THE CARDS THAT TOUCH!
Your objective is to communicate with your partner so he or she will know what card in that suit to return to you when they get in.