If the Commons has disagreed with the Lords about anything, it sends them a Message, carried by a Clerk in normal (non-pandemic) times, explaining what it has done. The Lords then consider that Message. If they do anything other than accept everything the Commons demands, they send a Message back telling the Commons where the areas of agreement and disagreement lie.

This is where a constitutional convention – not a law, or even a Standing Order – known as 'double insistence' comes into play. This convention holds that if the Lords say something must be done, and the Commons refuse to do it and offer no compromise, and then the Lords insist on their original change and the Commons insist on their original rejection, the whole Bill is dead.

There is a strong incentive to avoid 'double insistence', in order to save the affected Bill.

One of the most cunning ways of avoiding 'double insistence' is 'packaging'.

This is a way of putting a spin on the table tennis ball when you return it to the other House.

'Packaging' comes into play only after the first rally between the two Houses has finished. In other words, the Commons agrees a Bill; the Lords amend it; the Commons reject some or all of the Lords amendments ('ping-pong' has started); and the Lords do not fold (although this is the point when they generally do), but instead bat the ball back over the net to the Commons.

Up to this point, the rules have been fairly clear: there are a series of propositions (the Lords amendments) which the Commons must deal with one-by-one.

However, if the Lords send another Message back to the Commons, containing further disagreements but offering compromises, "it must be remembered" – as Erskine May quaintly puts it – that ...

"... there is no binding rule of order which governs these proceedings in either House...".

In other words, at this stage we are making it up as we go along.

This is where 'packaging' comes in.

Like some elaborate derivative of 'Go' or chess, or perhaps more like 'Minesweeper', the point at this stage is to avoid 'double insistence'. If you fail, you kill the Bill. So you:

• bundle up your replies and proposals to the other House in elaborate parcels to disguise so far as is possible the fact that you haven't agreed with it;

• put these packages in a Message;

• tie it up with a red or a green ribbon (oddly known in the trade as 'ferret'), depending on whether you are in the Lords or the Commons; and

• send it back.

(I have rather lost sight of the 'ping-pong' metaphor now.)

There is no recognised limit to the number of exchanges that are possible.

Usually, the Lords blink after the fourth or fifth exchange, acknowledging their subordinate status as the unelected House, and recognising that ultimately the Commons could send the Bill to the Lords a second time in the succeeding session, and (as long as it is the same text, and at least a year after the first occasion) it could then be passed under the Parliament Act procedures without any Lords input.

Sometimes, there is a recognition that a stalemate has been reached even without 'double insistence', and the House in which the ping-pong ball landed last simply walks away with it, and the Bill dies when the session ends with prorogation.

The issue with the Commons debate on the Lords Message on the Trade Bill on 9 February related only indirectly to 'packaging'.

The Lords had inserted a new clause ('Alton 1', after its instigator Lord Alton) in the Trade Bill giving the courts a power to strike down UK bilateral trade agreements done with governments which had been found to have committed genocide. The Commons rejected this (as Lords amendment 3) on 19 January. The Lords offered an alternative ('Alton 2', or Lords amendment 3B) which in essence restated their original clause with more skilful drafting and respect for the separation of powers.

Meanwhile, on 19 January the Commons had also rejected another Lords proposal ('Collins 1', after Lord Collins, numbered as Lords amendment 2) to involve select committees in assessing whether trade agreements met human rights standards. When the Bill went back to the Lords, the Lords offered an alternative ('Collins 2', or Lords amendment 2B), with a focus on crimes against humanity.

When these two Lords amendments went back to the Commons on 9 February, the government adopted and took charge of a 'package' from the Chair of the Justice Select Committee, Sir Bob Neill, rejecting both 'Alton 2' and 'Collins 2' but offering a compromise on select committees and genocide. With the government now in charge, this conferred procedural advantages on the motion to agree this compromise – most importantly, the chance to be decided after the expiry of the brief one-hour debating time allowed by the programme order the House had agreed on 19 January.

Sir Iain Duncan Smith MP tabled an amendment to the Neill (now government) compromise which would have moved the text back in the direction of the Lords' tougher proposals. However, as the programme order only allowed an hour for consideration of a Lords Message, time ran out. And because the programming procedure under Standing Order No. 83G privileges only government business after time is expired, the Duncan Smith amendment was not able to be moved.

This is where a combination of the privileging of government business, the ability of the government to decide how to package up its responses to the Lords, and the absence of a "binding rule of order" for considering Lords Messages, interacted to the government’s advantage.

Had the House voted against the government/Neill package, the selection list published on the day indicated that the Chair’s interpretation of what happened next was that the parcel would have become untied, and decisions would have had to be taken on 'Alton 2' and 'Collins 2' outside the package. However, the same Standing Order No. 83G meant that under programmed procedure those decisions would have been rolled up into one, meaning that the House would have been offered the choice of agreeing to both Alton and Collins, or disagreeing to them both: the decisions could not have been disaggregated. As Collins was a Labour frontbench proposal, there would probably have been considerable reluctance amongst the rebellious government backbenchers in the Commons to vote for it, even though not doing so would have meant losing Alton as well, along with the rejected Neill compromise. And they had no chance to vote for the Duncan Smith strengthening of the Neill compromise at all.

As it was, the House narrowly supported the government package, and that proposal has now gone back to the Lords – where, since there are no time limits on debate, it would not be surprising if in the next round on 23 February the Lords added in something along the lines that Iain Duncan Smith had tried to insert.

'Ping-pong' on the Trade Bill could thus go to another rally.

'Alton 1' was defeated in the Commons on 19 January by 319 to 308, a strikingly narrow margin for a government with a majority of 80. The government’s adoption of the Neill 'package' sidestepped a straight up-or-down vote on 'Alton 2', much to the chagrin of its supporters in the Commons; but, even so, the single motion (packaging disagreement to 'Alton 2', disagreement to 'Collins 2' and the Neill compromise) was passed on 9 February by only 318 to 303.

It is definitely worth tuning in for further rallies in this match.

This kind of high-skill 'ping-pong' has risks.

In 2004, the 'packaging' on the Planning and Compulsory Purchase Bill became so overwrought that the Lords detected that the Commons had inadvertently committed 'double insistence'. They passed a motion to allow themselves to continue the 'ping-pong' anyway.

Back in the day, rather than play 'ping-pong', the two Houses would meet in a 'conference' to try to thrash out their differences. The last one was held in 1836. (Curious fact: when the new Palace of Westminster was designed in the 1830s-1840s, the rather grand room that now serves as the Members’ Dining Room was intended as a location for these conferences, but there has never been one held there.) Given the potential for opacity of the messaging system which replaced them, maybe we should consider their revival.

It might be easier to follow the 'ping-pong' live, rather than rely on Messages emerging from the room telling us what each player had just done with the ball.

Conferences still survive in the US Congress, along with other 18th-century Westminster procedures long regarded as obsolete in this country – like impeachment.