The purpose of middle-up-down is to distinguish 3-card holdings from doubletons. If partner sees you lead 9-then-7, partner knows you have led a doubleton. If you lead 7-then-9, partner knows you have another card in the suit... with 9-7 only, you would have led the 9 first
The middle card is chosen rather than the lowest from three rags, so that the lead of the lowest card in a suit will promise an honour card in the suit led. In standard methods, there are several valuable inferences when you can tell that partner has led the highest spot card or the lowest spot card possible.
Lead of the highest top spot card is normally a singleton or a doubleton. (Exceptionally, it may be bottom from a holding like K-J-x.)
Lead of the lowest spot card promises a singleton or an honour in the suit led. (Lowest cannot be top from a doubleton or middle from M. U. D. It is either a singleton or from an honour card holding)
High spot card followed by lower = doubleton.
Low spot card followed by higher = at least one more card in the suit led.
With M. U. D. you cannot tell the number of cards held by the leader until the second round of the suit. The first lead is bound to be ambiguous. The benefit is that top card leads and bottom card leads are clarified. Pairs who lead lowest from three rags believe it more useful to indicate the number of cards held on the first lead of a suit than to promise an honour (perhaps only the 10 or jack) with the bottom card lead
From 10-7-3 and equivalent holdings, the bottom card is led. The 10 will often be too valuable to discard on the second round of the suit.
|9 6 4|
|10 7 3||K Q 8 5 2|
West leads the 3 to the queen and ace. Later, East leads the king. West plays the 7 and keeps the 10 to capture the 9 in dummy. West has led 3-then-7 so that East can work out that West has the 10 left. If West leads the 7 first - queen - ace, what does West play when East later cashes the king? If West drops the 10 to follow through with middle-up, the 9 in dummy is high, so that is out, but if West plays 7-then-3, East will read the lead as a doubleton.
|9 5 4|
|10 7 3||A K 8 6 2|
West leads the 3 to the king and drops the 7 under the ace. West's 3-then-7 lets East know that the missing card in the suit is with West. If West wrongly leads the 7 first, which card does West play when East takes the ace on the next trick? To drop the 10 would set up dummy's 9. However, if West drops the 3, West has led 7-then-3 and East may be deceived into thinking West can ruff the third round of the suit. A third round of the suit could be fatal for the defence.
|10 5 2||K 8 7 6 4|
|A 9 3|
West leads the 2 to the queen, king, ace. When the jack is played later, West follows with the 5 and East knows West has another card in the suit. The same follows if East ducks the first round and plays the king next time. However, if West leads the 5 first, he has to play the 2 next time as he cannot afford the 10. 5-then-2 shows a doubleton and East may now abandon the suit, fearing South has the strength.
Note: If West wrongly leads the 10 (the dreaded top-of-nothing) in any of the above layouts, this will create an extra trick for declarer.
The 9 lead in standard methods will always be a singleton or a doubleton (with one rare exception). The 9 cannot be M. U. D since if the 9 is' middle', the 'up' card would be an honour and we never play Middle-Up-Down from a suit including an honour. The rare exception is from K-J-9, the standard lead is also the 9.
Where the 8 is led and the 9 is visible, the same inference applies. The 8 will be a singleton or a doubleton. Since the 9 is visible, the 8 cannot be a Middle-Up-Down lead, for again the 'up' card would be an honour. The rare exceptions: The 8 is also the standard lead from K-J-8, K-10-8 and Q-10-8.
The same inferences apply if the 7 is led with the 8 and 9 visible, or the 6 is led with the 7,8,9 visible.
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