Various grains (which may be malted) are used for different varieties, including barley, corn (maize) , rye, and wheat.
How Scotch Whisky Is Made is a film taking you through the production process for Scotch Malt Whisky, Grain Whisky and Blended whisky. The first part of the film takes you through the production process for Scotch Malt Whisky from malting, mashing, fermentation, distillation and finally maturation.  As you said, whiskey may be made from either wheat, corn, rye or barley. American bourbon is predominantly corn, Canadian rye from rye, Scotch from barley & Irish from any of the aforementioned grains. Whiskey made from barley is tasteless, rye, bourbon impart good flavour. In American Rye whiskey must be made from a mash made from no less than 51% rye. The other ingredients commonly used include corn and barley. Same as Bourbon it must be aged in charred new oak barrels distilled to an ABV less than 80% (and like bourbon it must be no more than 62.5% when added to the cask). If you're a whiskey (and especially bourbon) drinker, you need to know what that means. By definition, whiskey (or whisky, in Scotland) is a spirit distilled from fermented grain mash — grain varieties include wheat, rye, barley, and corn — and then aged in wooden barrels. Whiskey is made all over the world and there are many popular styles including Scotch whisky, Irish whiskey, and American whiskey. The most popular form of American whiskey is bourbon, which has its own specific definition.  The world of whiskey is a complex maze of grains, flavors, and distillation processes. Essentially, all whiskies are distilled from malted grains such as barley, corn, rye, wheat, etc. Considered to be the father of all whiskey, Irish Whiskey is often a blend of pot-stilled malted and unmalted whiskey and column-stilled corn-based grain whiskey or, as in the case of Bushmills, triple distilled malted barley (single malt). Scotch is whisky made in Scotland, while bourbon is whiskey made in the U.S.A, generally Kentucky. Scotch is made mostly from malted barley, while bourbon is distilled from corn. If you’re in England and ask for a whisky, you’ll get Scotch. But in Ireland, you’ll get Irish whiskey (yep, they spell it differently for a little colour). Whisky or whiskey is a type of distilled alcoholic beverage made from fermented grain mash. Various grains (which may be malted) are used for different varieties, including barley, corn (maize) , rye, and wheat. Whisky is typically aged in wooden casks, generally made of charred white oak. Whisky is a strictly regulated spirit worldwide with many classes and types. The typical unifying characteristics of the different classes and types are the fermentation of grains, distillation, and aging in wooden barrels. 1 Blended malt whisky is a mixture of single malt whiskies from different distilleries. 2  If a whisky is labelled pure malt or just malt it is almost certainly a blended malt whisky. 3  This was formerly called a vatted malt whisky. 4  Blended whisky is made from a mixture of different types of whisky. Scotch whisky, often simply called Scotch, is malt whisky or grain whisky made in Scotland. Scotch whisky must be made in a manner specified by law. All Scotch whisky was originally made from malted barley. Commercial distilleries began introducing whisky made from wheat and rye in the late 18th century. All Scotch whisky must be aged in oak barrels for at least three years. Any age statement on a bottle of Scotch whisky, expressed in numerical form, must reflect the age of the youngest whisky used to produce that product. A whisky with an age statement is known as guaranteed-age whisky. Made primarily of corn or wheat and supplemented with rye, barley, or barley malt, Canadian whiskey is aged in used oak barrels for a minimum of 3 years although most are aged for 4-6 years. Almost all Canadian Whiskey is a blend of various grain whiskies of different ages. Considered to be the father of all whiskey, Irish Whiskey is often a blend of pot-stilled malted and unmalted whiskey and column-stilled corn-based grain whiskey or, as in the case of Bushmills, triple distilled malted barley (single malt). Four countries produce distinctly different whiskies: Ireland (Irish Whiskey), Scotland (Scotch), America (Bourbon, Tennessee Whiskey, Rye Whiskey, Blended American Whiskey), and Canada (Canadian Whiskey), although whiskey is made throughout the world. Considered to be the father of all whiskey, Irish Whiskey is often a blend of pot-stilled malted and unmalted whiskey and column-stilled corn-based grain whiskey or, as in the case of Bushmills, triple distilled malted barley (single malt). Scotch whiskey is only made in Scotland, and it is always made as single malt from 100 percent malted barley in a specially shaped still. Understand the specific characteristics of scotch whiskey with information from an experienced distiller in this free video on alcohol and liquor. 
http://www.youtube.com/watch?v=7VSj9xaNWpw http://www.realmendrinkwhiskey.com/know-your-whiskey-the-difference-between-bourbon-and-scotch/ http://www.mensjournal.com/food-drink/drinks/what-exactly-is-the-difference-between-bourbon-and-whiskey-20140905 http://cocktails.about.com/od/Whiskey-Information-and-Reviews/ss/Whiskey-Wisdom-Understanding-the-Basic-Styles-of-Whiskey.htm http://mentalfloss.com/article/30278/what%E2%80%99s-difference-between-scotch-whiskey-and-bourbon https://en.wikipedia.org/wiki/Whisky https://en.wikipedia.org/wiki/Scotch_whisky http://cocktails.about.com/od/Whiskey-Information-and-Reviews/ss/Whiskey-Wisdom-Understanding-the-Basic-Styles-of-Whiskey.htm http://cocktails.about.com/od/Whiskey-Information-and-Reviews/ss/Whiskey-Wisdom-Understanding-the-Basic-Styles-of-Whiskey.htm http://www.youtube.com/watch?v=hEPlt88bFhQ
what is whisky made of