I was just surprised to hear that it was primarily made from Barley like single malt, but not malted!
A small percentage of malted barley is
used, but this is chiefly to assist with kickstarting the fermentation process. Very little unmalted barley is used, except for in Ireland, where unmalted barley is the basis of Potstill.
The rest of the mashbill in grain whisky is chiefly either maize (corn) or wheat.
Now up until recently, maize was the more common (and certainly preferred) ingredient of the two, however, I understand that certain economic pressures now apply with the EU. Subsidies & concessions now make wheat the cheaper (and therefore more common) ingredient for grain whisky.
One distiller recently told me that he much preferred to use maize - it gave a cleaner, sweeter, flavour, and was also apparently much cleaner to use (i.e. less residue, bulk, and easier fermentation), but that he was resigned to using wheat because of the EU pressures.
Grain whisky is often unfairly described as being 'neutral', but of course it has flavour. Grain whisky is quite uncommon in bottled form. Some of the independent bottlers have bottled some delicious grain whisky (I'm thinking specifically of Duncan Taylor's 39yo Invergordon), but Diageo do actually bottle their single grain whisky from the Cameron Bridge distillery (it's branded as Cameron Brig). I've tried this, and it's very smooth & sweet.