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They are the same grape!
However some winemakers would argue that, stylistically speaking, Shiraz and Syrah are not the same. Syrah might be used to denote a more restrained ‘old world’ style of wine, for instance.
But be warned; there are no rules to govern this distinction and such a crude demarcation cannot be so easily drawn.
A traditional Shiraz from South Australia would be considered bolder, with riper and more concentrated fruit, perhaps with earthy and dark chocolate notes plus some extra spiciness from the use of new oak. One might also expect higher alcohol levels, given the hotter climate.
Syrah wines from the northern Rhône can also have a robust structure with dense dark fruit, but one might classically expect a leaner, more austere character alongside greater prominence for floral aromas and black or white pepper.
However, you’ll probably know by now that the wine world loves to defy sweeping generalisations.
You can find Syrah / Shiraz wines in both southern France and Australia that place greater emphasis on luscious, plummy fruit and are subsequently more approachable at a younger age. Likewise, you’ll find Syrah and Shiraz-labelled wines that are more structured and built to last.