Number positions consistently before comparing drops
For this example, call the columns A through G from left to right and count heights upward from the bottom. Height one is the lowest row and height six the highest. Those labels describe the example; the real game still uses seven columns and six rows with ordinary gravity.
An empty column receives a piece at height one. If it contains two pieces, the next piece lands at height three. You cannot choose height four while height three is empty. A diagonal drawn above a short stack may look almost complete while its missing square remains unavailable to the next drop.
Construct the support case using legal turns
On the shared two-player board, make these alternating choices: X in B, O in A, X in C, O in B, X in C, O in C, X in D, and O in D. It is now X’s turn. O occupies A1, B2, and C3, which are three steps along a rising diagonal.
Column D contains X at height one and O at height two. The diagonal’s missing square is D4, not the currently reachable D3. The sequence is a constructed human-controlled example, not a claim that the main computer voluntarily chooses all those moves. Its purpose is to make the supporting pieces visible and the next landing height checkable.
A piece below the gap can create the winning height
If X now drops in D, that piece lands at D3. It does not occupy O’s diagonal completion square at D4. On the following turn, O can drop into D4 and connect A1, B2, C3, and D4. The intervening X becomes part of the supporting stack underneath O’s winning position.
This shows why the instruction “play in the threatening column” is incomplete. You must know which row that play reaches. A piece is not a block simply because it is somewhere below a line’s gap. Inspect the board after your proposed action and ask which square the opponent can legally occupy next.
Separate the local demonstration from a complete solution
The example proves one consequence of X choosing D at that position. It does not claim that every other column saves X, identify an optimal complete strategy, or show that every opponent follows a fixed sequence. Those questions would require considering the other threats and legal replies as well.
Before defending any diagonal, first check whether you have an immediate legal win of your own. If not, identify the opponent’s actual closing square and available support. If two different winning drops are already available, one defensive piece may not solve both. Do not invent a floating piece or two drops in one turn to rescue an attractive plan.
Keep the playable format and the ending clear
The two-player variant waits for human X and O inputs on the same device. It is useful for reconstructing positions without an automatic reply changing your chosen sequence. The main computer mode instead supplies the opponent’s move locally. Neither format is an online room unless a separate feature explicitly says so.
A winning drop ends the round as soon as four consecutive matching pieces form a horizontal, vertical, or diagonal line. Do not add another defensive move after O completes D4 in the example. You can pause or start again through the confirmed control if you want to inspect another continuation. Local saving depends on the browser retaining data, not on an account that synchronizes every exercise. The transferable rule is exact: include the supporting stack when deciding whether a diagonal square is reachable now.
Frequently asked questions
Can I drop directly into a high diagonal gap?
Only if the existing stack makes that square the lowest open space of the column.
Why does X in D fail in the example?
It lands at D3, supplying support for O to occupy the actual diagonal gap at D4.
Does the example prove the best move for X?
No. It isolates one consequence and does not solve every alternative continuation.
Who controls O on the board used here?
A person does. The same-device two-player variant does not supply an automatic reply.
Can X block after O completes the diagonal?
No. A valid four-piece winning line ends the round immediately.