This page usually contains a piece of theory, in this case about 'Blockage' , including an example (or two).
Three exercises will follow, one in each of the next three weeks (click 'Varia***' ).

Sometimes we cannot cash all our tricks in a suit at once, despite having all high cards in that suit, because of a blockage. For instance the bare ♥AK opposite ♥QJ109 should in principle yield four tricks but after having cashed the ♥AK we cannot immediately cash the remaining two tricks.

Two items of advice in case one of our own suits is blocked:
1. Retain entry cards in the hand containing the high cards which cannot be cashed at once.
2. Unblock the suit as quickly as possible.

Take this deal, from a team match (IMP scoring therefore), meaning overtricks are not important:

N/EW		♠	A J 10
		♥	J 5 3
		♦	A K 5 4
		♣	K Q J
		♠	K 6
		♥	9 4 2
		♦	8 7 6
		♣	A 8 7 6 4

West leads the ♠3 (third or fifth best).

Because of this lead declarer can make sure of three spade tricks and many declarers would quickly instruct dummy to play 'a small one'.
Often they would get away with it, but not this time:

N/EW		♠	A J 10
		♥	J 5 3
		♦	A K 5 4
		♣	K Q J
♠	9 7 5 4 3 2			♠	Q 8
♥	A 8 7 6			♥	K Q 10
♦	J 2			♦	Q 10 9 3
♣	3			♣	10 9 5 2
		♠	K 6
		♥	9 4 2
		♦	8 7 6
		♣	A 8 7 6 4

East covers with the ♠Q and South has to win (if he doesn't EW can cash four heart tricks). On the second club the 4-1 split is unveiled. The contract is doomed since the suit is blocked: after cashing North's third club South lacks an entry card to cash the fourth and fifth club. He is left with eight tricks: three spades (sure enough), three clubs and two diamonds.

He has paid the price for ignoring advice #1 by squandering South's sure entry for the fourth and fifth club, the ♠K. (Of course he also made these standard errors: he played too quickly to trick one, he played without making a plan and he did not count his tricks.)
Declarer can see two diamond tricks (the ♦AK) so if (!) he makes five club tricks, he needs two spade tricks only. Therefore he should win the first trick with dummy's ♠A and then at once unblock North's clubs. Next he crosses to his carefully kept ♠K to cash the remaining clubs. Nine straight tricks without finessing or losing the lead.

In defence blockages are a recurring theme as well.
An example, again from a team match:

N/NS		♠	A 6 4
		♥	A 5
		♦	Q J 10 7
		♣	8 5 4 3
		♠	K Q 3
		♥	10 9 7 4
		♦	A K 8 3
		♣	K Q 6

West leads the ♥3.

South can count eight top tricks: three spades, a heart and four diamonds. Effortlessly he can establish one more in clubs, so the only danger is that EW, gaining the lead with the ♣A, cash four (or more) hearts.

Therefore the contract is in danger if the heart suit is 5-2 or worse.
Declarer might duck the first trick. EW will continue the suit, South winning with the ♥A. He plays a club and makes his contract if either the heart suit is 4-3 (in that case the contract was safe from the start) or if the player with the ♣A had two (or fewer) hearts (he is now out of that suit).
The reasoning is correct.

Declarer can do better though. He should realise that West, in view of his lead of a small heart, does not have the ♥KQJ. So East has one heart honour at least. If the suit is 5-2 or 6-1 (that is when the contract is in danger) East began with that honour card bare or doubleton.
Therefore declarer should go up with the ♥A at trick one!

		♠	A 6 4
		♥	A 5
		♦	Q J 10 7
		♣	8 5 4 3
♠	9 2			♠	J 10 8 7 5
♥	K J 8 3 2			♥	Q 6
♦	6 5 2			♦	9 4
♣	A 9			♣	J 10 7 2
		♠	K Q 3
		♥	10 9 7 4
		♦	A K 8 3
		♣	K Q 6

If East plays the ♥Q under the ♥A - whether he does it voluntarily (as he could do here in order to unblock), or is forced to do so (if that card would be bare, unlike here) - South gets a second heart guard.
If East does not play the ♥Q under the ♥A, the suit is blocked for EW: declarer can establish his club trick at leisure.

Note that the first line of play fails here. East wins with the ♥Q and returns the ♥6. Since dummy's ♥A beats air, West can retain both honour cards. Later he gains the lead with the ♣A, and cashes three more hearts for down one.

Try the three 'Exercises on Blockage' in the three weeks to come.