Today's budget has been billed as the most important since devolution in 1999.

Why? Because Finance Secretary John Swinney will for the first time set a Scottish Rate of Income Tax (SRIT).

Under powers devolved in the 2012 Scotland Act, Holyrood will raise about half the income tax paid out in Scotland.

The revenue will become part of the Scottish Government's budget, rather than going to the Treasury for UK spending.

The Treasury's "block grant" to the Scottish Government, its annual budget hand-out, will be reduced accordingly.

The system works like this: UK income tax will be cut by 10p across the basic (20p), higher (40p) and additional (45p) rates.

Mr Swinney will decide today whether to make up the shortfall exactly, by levying an SRIT of 10p, or seek to raise extra revenue by putting up the rate, or ease the burden on taxpayers by cutting it.

Sources have indicated he plans to set a 10p rate, effectively maintaining existing UK income tax rates.

He has long argued the SRIT power does not allow him to change tax progressively.

In other words, he cannot choose to increase tax on the better off but reduce it for the less well off, as the SRIT must apply across the three bands.

The system is likely to be superceded for the following financial year, 2017/18, when the new Scotland Bill, now going through Westminster, will devolve almost complete control over income tax to Holyrood.