Welcome to the devolution dance: an elegant gavotte between Holyrood and Westminster.
The object? In part, it is believed, to ensure that substantial advance is made with the Scotland Bill in the Commons before MSPs arise from their law-making toil for the troublesome business of seeking re-election.
Why? In order, arguably, to enhance the status of the offer to counter the SNP project of fiscal autonomy and independence: to stress the progress recorded.
According to taste, the bill itself is either a significant enhancement of Holyrood's financial accountability or a back-door device to impoverish the Scots or a missed opportunity.
It is receiving its Second Reading in the Commons today in the aftermath of (occasionally contentious) scrutiny at Holyrood. Which is where the concept of choreography comes in.
The constitution of the United Kingdom is explicity reserved as an issue for Westminster to handle in Schedule V of the Scotland Act 1998. (I know, I know, but bear with me.)
The new Scotland Bill builds upon that 1998 Act: altering the balance of reserved and devolved powers and amending the arrangements by which Holyrood is financed.
But the constitution of the United Kingdom remains reserved. The Scotland Act 1998 is a Westminster measure. It lies in Westminster's hands to change it.
Actually, everything still lies in Westminster's hands - but, by convention, they stay away from the devolved stuff.
I well recall the anguished debates in the Constitutional Convention as to how the devolution settlement could be entrenched against future predations from Westminster. The conclusion?
Or, at least, it was concluded that any such entrenchment was political rather than constitutional. On the lines of "they wouldn't dare."
That was formalised a little through the device which came to be known as the Sewel convention, after the estimable and droll Lord Sewel, then a Scottish Office Minister who proposed that Westminster should not legislate in devolved areas without prior consent from Holyrood.
But only a little. Westminster remains sovereign, Holyrood remains its devolved creature.
Sensibly from its perspective, Westminster avoids stressing this relative status. Unionist politicians at Holyrood comply. Nationalist politicians raise the issue regularly.
Which brings us (patience, patience) back to the present Scotland Bill.
How does Westminster alter the constitutional arrangements in the Scottish Parliament without highlighting Holyrood's subordinate status overmuch?
Answer: that gavotte, the reform reel. Will you, won't you, join the dance?
And so the Scotland Bill is first subjected to debate at Holyrood, not Westminster.
On the 9th of December, Labour, Conservative and Liberal Democrat MSPs combined to support the general principles of the bill.
Incidentally, the phrase "general principles" quite deliberately echoes the term used when, as today, MPs give a Second Reading to a piece of legislation at Westminster.
Onto the next phase.
A special committee under the convenership of Wendy Alexander has been taking evidence on the Scotland Bill prior to preparing a report.
They began work ahead of today's Second Reading.
It is anticipated that the Alexander committee will produce their report before Westminster begins its committee stages on the bill.
Those will be taken over three days on the floor of the Commons, this being a constitutional bill.
Then it is envisaged that there will be a formal Legislative Consent Motion at Holyrood (an LCM or Sewel motion).
Without that, says the Scottish Secretary Michael Moore, the Commons would not proceed to final approval.
Views arising. The UK government and parties adhering to the Union in the Scottish Parliament say that it is a deft and sensible way of ensuring that Holyrood is fully involved in scrutinising what remains a Westminster Bill.
Mr Moore made such points on Good Morning Scotland this morning.
The alternative perspective, from Nationalists and others supporting fiscal autonomy, is that the Bill is receiving insufficient scrutiny in relation to what are substantial changes to the financial regime (which they dislike for a range of reasons.)
That is, partly, why such a fuss was raised over the reception afforded by the committee to Professors Andrew Hughes Hallett and Drew Scott.
Yes, it is argued, it was reasonable to question them on their advocacy of fiscal autonomy but they should have been questioned first on the principles of the Bill before the committee.
In response to the general point about scrutiny, those advocating the bill say that the issue was extensively discussed by the Calman Commission and its offshoot teams, that it is being examined at Holyrood and that it will undergo a full legislative process at Westminster.
Further, those advocating the bill say that the SNP complaints about scrutiny are bogus: that they want to block the bill, not study it.
In response, Nationalists insist that they welcome enhanced powers for Holyrood - but that the financial proposals are misplaced.
Among all the other issues, this will be a choice to be placed before the people.
Not just at Westminster, not just at Holyrood, but at the forthcoming election.