It was past the morning rush and my pre-caffeinated eyes coasted halfway through the coffee menu at Blue Bottle Coffee ‘s new Brooklyn roastery before getting stuck onto a road barrier listed only as SG-120. I shifted my gaze into reverse, spotted the familiar signposts espresso and macchiato and tried to make use of all available evidence: If, as I suspected, SG-120 referred in some way to a Japanese video game console, what made it worth an additional 25 or 50 cents?

The barista, bless her heart, did not roll her eyes when I asked her to explain the connection between sega genesis and espresso, nor did she make a joke about it being a tattoo removal cream. The SG-120, she explained, was the model number of the small, delicate Japanese glass in which Blue Bottle served its single-origin mini-lattes. (So I was right: it was Japanese!) Whereas the similarly proportioned coffee made with Retrofit, the house espresso blend, is called a Gibraltar, after the classic Gibraltar glass, those made with beans from a single growing region – in this instance Kintamani, Bali – took the shape and name of a different receptacle.

The SG-120 is printed on the Blue Bottle menu, unlike the unlisted Gibraltar, but it too enjoys a cult status and purposefully obscure origins. Excitedly unpacking a large shipment of brewing equipment and supplies from the highly respected and now wildly popular Japanese glassware company Hario at Blue Bottle’s location in San Francisco’s Mint Plaza (see map), owner James Freeman and his colleagues discovered that the pieces they’d ordered as water glasses were much too small and delicate for that rough-and-tumble purpose. (They probably wanted the SG-300 rock glasses or HPG-300 tumblers). The Hario SG-120s are amongst the world’s lightest, thinnest-lipped, shortest-bottomed shot glasses. They’re perfect for sake.

So what does a coffee bar do with dozens of ethereal sake glasses it can’t use? Freeman observed their 120-millitre (about 4oz) volume was roughly equivalent to that of the Gibraltar glass he was using and thought the SG-120 could be employed to visually differentiate the single-origin Gibraltars from those made from an espresso blend. His wife Caitlin inspected the boxes of Hario glasses and suggested they name the drink after the model number printed on them.

“We couldn’t call it a Gibraltar because it’s not in a Gibraltar,” says Freeman. “That would be wrong.”

An early review of the SG-120, from TheShot, a San Francisco-based coffee blog, was unfavourable: “It felt cheap and almost disposable, and its thinness and materials added no real thermal properties.”

Fair enough. That view, however, overlooks an elegance and simplicity that adds refinement to a quarter-pint latte. When you look at the SG-120 from the side it appears to float. Instead of the liquid taking its shape from its container, your impression is of a glass taking its shape from the swirls of textured milk and espresso within it. Moreover, if the glass is too hot to hold, that reflects a problem with the SG-120’s preparation: The milk should never be that hot.

My affection for the SG-120, especially now that I know what it is and know that it’s safe to drink, is shared by Blue Bottle habitués on both coasts.

“It’s the most stolen item in the shop,” boasts Freeman.