“The ayes to the right...” That’s how Speaker of the House of Commons John Bercow will begin announcing the result of the vote that could determine Britain’s future.

But amid the flummery of the British Parliament, where members cannot address each other by name, and the security is provided by men in tights, how can you tell who’s actually won and lost? Even veterans of the system can be caught out by procedure. Here’s our guide to Tuesday’s vote.

The Motion

Last year Prime Minister Theresa May agreed to give Parliament a “meaningful vote” on the deal, in a concession to lawmakers to get her flagship Brexit legislation passed. The House of Commons doesn’t usually vote on international agreements, so the procedure in this case has been made up on the hoof. May has laid an eight-line motion saying that Parliament approves the deal.

Kick-off: 19:00

May will make the final speech, closing the debate in time for voting to kick off at 7 p.m.

Amendments First

Before they can vote on the motion, lawmakers decide if they want to modify it. There’s no limit to how many amendments will be voted on and it’s up to Bercow to decide which ones to put to a vote. He might group complementary ones together for a single vote. You can see all the possible amendments on the order paper, which is updated each morning, under “Business of The Day.” To find out which amendments have been selected, check the House of Commons Twitter feed, or the Labour Whips feed, which is often more informative.

‘The Question Is...’

Bercow will announce what they’re voting on. Amendments for this vote are being identified by letter.

‘As Many As Are Of That Opinion...’

Bercow will then invite supporters of the amendment to shout “Aye!” and opponents to shout “No!” If either side doesn’t shout, the other side wins by default.

‘...Say Aye!’

Which side wants what? In this case, amendments will mainly come from May’s opponents. So the government will mostly be voting No to them -- it doesn’t want the motion amended. But be alert to the possibility of a government-backed amendment aimed at buying off opponents. You can spot these by the people supporting the change on the order paper -- a minister’s name is a giveaway.

‘Division! Clear the Lobby!’

Assuming there’s a shout of “No,” Bercow announces a vote. In the House of Commons, members vote by walking through one of the two lobbies on either side of the chamber. The entrances are at opposite ends. On television, voting is always shown as a wide shot of the whole chamber, looking toward the Speaker’s chair. The “Ayes” will walk away from the camera, out of the door behind the chair. The “Noes” will walk toward the camera.

Members of Parliament have eight minutes from the moment that the Speaker announces the vote to get into the right lobby. A bell sounds across the Parliamentary estate, and elevators are reserved for members during votes. Party whips stand at the entrance to each lobby, attempting to direct their flock through the correct door and intercept and dissuade members going the wrong way.

Meanwhile at the exit to each voting lobby, tall wooden desks are slid into place at which clerks sit, ticking off the names of members as they file past. This is now done on iPads, but that data isn’t used to tally the results of the vote. That job is done by the tellers. Remember them, they’re important.

‘Tellers for The Ayes?’

A minute or so after Bercow has announced the vote, he’ll stand up and announce it again. This time, he’ll also announce two lawmakers from each side of the vote to act as tellers. One from each side now heads to the exit of each lobby. Their job is to count members leaving. They do this out loud, without the aid of technology. Sometimes it goes wrong. One Labour member once voted three times without being spotted. Occasionally members walking past the tellers will amuse themselves by shouting out other numbers, in an effort to confuse them.

‘Lock the Doors!’

While some whips are manning the lobbies, others are conducting a sweep of parliament’s bars and toilets, looking for stragglers. This isn’t a job without risk. In one recent vote, the Conservative whip charged with minding the party’s heavier drinkers in the bar misjudged his own capacity for alcohol, and ended up being carried through the voting lobby by the man he was supposed to be minding.

When eight minutes have passed, Bercow shouts “Lock the doors.” The whips duck into the lobbies and officials lock the doors behind them. If a member isn’t in the lobby now, it’s too late for them to vote. And if they’re in the wrong lobby, having ducked in to use the toilet, for instance, they’re stuck.

At the far end of the voting lobby, members split into queues, depending on the first letter of their name. Because people higher up the alphabet are more likely to be elected -- they appear higher on British ballot papers -- the queue for “N to Z” is generally shorter.

Having been checked off, members file past the tellers -- the government whip will be on the right, counting, while the opposition whip on the left is there to confirm -- and then back into the chamber.

It generally takes about seven more minutes to get a result. The last person through the lobby is the whip who was standing on the door. They check the toilets in the lobby and make sure that everyone has voted, and then vote themselves. “All out,” they tell the tellers as they walk past.

‘Order’

The tellers now meet in the middle of the chamber, compare their tallies, and prepare to announce the result. They’ll stand four across. If you’ve remembered who the tellers for the ayes and noes are, then you can tell the result at this point -- the winning side stand on the Speaker’s left. The camera will cut to show them face on. Members in the chamber -- who know whose tellers are whose -- may well cheer at this point. But beware: Government tellers are used to standing on the winning side, and sometimes forget. So tellers have occasionally swapped sides at the last minute.

‘The Ayes to the Right’

The tellers now take a step forward, and a teller for the winning side will read out the result. They always read the “Ayes” number first. If this number is more than 318, the Ayes have almost certainly won -- but not definitely, because some lawmakers mark their abstention in a vote by voting in both lobbies. You can only be sure when the “Noes” number has been read out.

‘The Noes Have It!’

The teller hands the result to a clerk, who hands it to Bercow, who reads it out again, and then adds which side -- the Ayes or the Noes -- “have it”.

‘Unlock!’

Bercow orders the voting lobbies to be unlocked. The vote is over.

So... Who won?

Most of the amendments will be hostile to May’s deal, so if any of those passes she’s lost. If this happens, it’s possible the government may abandon its motion, and voting will stop altogether. But it’s equally possible that all of the amendments will be rejected, in which case, Parliament will start voting on May’s unamended motion. At that point, a win for the Ayes is a win for May.