When you're talking about base malt, it's useful to remember that in general it's ALL "Two-row". The types are differentiated by maltster and kilning. For example, golden promise is a UK two-row malted in Scotland, while maris otter is a British two-row malted in England. Castle pilsner malt is a two-row barley malted in Belgium, and so on.
I was reading up on this recently and thought I'd add to the conversation. Malt spec sheets can have some cool information.
Almost all base malts are blends of two-row Spring barley varieties, of which there are probably thousands. Maris Otter is an exception in that it is a Winter barley. Maris Otter and Golden Promise are also exceptions in that the malts are not blended; both are named for the single barley variety farmed for the malt. GP is interesting in that it's an X-men grain: formed by random mutation after exposing another barley to gamma radiation that gave it superpowers for farming and malting.
Blending barley is a safe way to get consistent malts, much like blending whiskeys to get more consistent flavors year-to-year over large batches. Also worth noting about blending barley is that the grains are often sourced from different countries. Your German malt may contain grains farmed in France, Belgium, and the Czech republic.
Most of this doesn't matter to the brewer as the key malt properties of color and flavor come not so much from the source grains but from kilning, which historically is regionalized. However, modern technology allows the large malt houses wide control over kilning techniques. Vienna, regardless of where it's malted is cured at low temperatures and slowly dried at high temperatures with high moisture as was traditionally done in Austria. Likewise, anyone's Pilsner is kilned to replicate classic bohemian maltsters, all Pales are kilned to replicate classic UK techniques. Every malt house gets slightly different results, so you'll find variations even in the same German Munich between Weyermann, Bestmalz, and Schill.
6-row is uniquely American. During the Industrial Revolution, the quest for profit aimed US brewers towards lower costs. 6-row was cheap and the extra diastatic power allowed them to substitute cheap adjuncts with low DP like rice (Budweiser) and corn (Yuengling). The Europeans distanced themselves from this process and don't malt with 6-row, so it's not farmed there.
