Lesson 2 of 4
Threats
5 min

Odd and Even Threats in Connect Four

Two players who always block can still lose. A threat that can't be played yet waits until the board fills up, and then counting the discs decides who has to drop into the hole under it.

Step 1 of 5

A threat that has to wait
A threat is an empty hole that would complete your four. Count the rows from the bottom, 1 to 6. Red's three on row 3 is blocked on the left by Yellow, so Red's threat is the starred hole at its right end, on row 3 of column 6.

Nobody can drop into it yet. Column 6 holds one disc, so the next disc there lands on row 2. If Yellow drops it, Red drops on top and wins. If Red drops it, Yellow drops on top and blocks. So both players stay out of column 6 and play somewhere else.
The board fills up
Now the board is nearly full. Red's only threat is again on row 3 of column 6, and Yellow has none. Watch the last four free holes outside column 6 fill up.

Then Yellow has nowhere else to go. Count the discs: the other six columns hold 36 and column 6 holds one, so 37 are in. Red drops the odd-numbered discs and Yellow the even-numbered ones, so the 38th, the one that lands on row 2, is Yellow's. Red drops on top of it and wins. That disc completes row 3 and a diagonal too, and the board lights the diagonal.

The same count works for any threat, as long as nobody makes another one. The disc that goes under a threat on an odd row is always Yellow's, and under a threat on an even row it is Red's. So Red wants threats on odd rows, and Yellow wants them on even rows.
Whose turn is it?
This time the threat is Yellow's: Yellow's three on row 3 makes the starred hole on row 3 of column 6 a Yellow threat. Column 6 holds one disc. Both players stay out of it, nobody makes a new threat, and the other columns fill up. Who has to drop into the hole under the threat?
Question
Make an odd threat
You are Red, and neither side has a threat yet. One disc makes a threat on an odd row that Yellow can't reach yet. Other discs make threats too, but on the wrong row, or where Yellow can block them at once.
Your move
Make a threat on row 3 or row 5, with an empty hole under it.
Make an even threat
Now you play Yellow, and again neither side has a threat. Yellow wants threats on even rows: 2, 4 or 6.
Your move
You are Yellow. Make a threat on an even row, with an empty hole under it.

What you learned

A threat is an empty hole that would complete your four. While the hole under it is empty, nobody can use it.
Nobody wants to drop under a threat. Under your opponent's, you lose. Under your own, your opponent blocks it.
When the other columns are full, counting the discs decides who has to drop under it.
Red wants threats on odd rows (3 and 5), and Yellow on even rows (2, 4 and 6).