The boundary is a document
The line between a core green tea region and the country around it is drawn on paper, not tasted in the cup.
For Longjing（龙井）the national standard GB/T 18650 sets out three production areas: Xihu（西湖）, Qiantang（钱塘）and Yuezhou（越州）.
Only tea from Xihu can carry the West Lake name, and that area is roughly 168 square kilometres of hills on the western edge of Hangzhou.
Qiantang covers the rest of the city and the counties beyond it. Yuezhou covers parts of Shaoxing and the hills east of the city.
Tea from all three may be sold as Longjing under the standard. Tea from outside all three may not.
This is worth knowing before you pay, because the difference between the first area and the second is mostly price.
The current version of the standard dates from 2008, and the three-area structure has held since.
Other Chinese teas take a simpler view and protect a single zone, which makes them easier to understand and far easier to police.
What the core gives you
What Xihu actually gives you is a head start rather than a different tea.
The hills hold heat, the soil is weathered sandstone, and the gardens sit low, mostly between 100 m and 300 m.
Low elevation means an early flush, and an early flush means the first picking lands before the spring rain.
The leaf is small and the shape is flatter than what the surrounding areas manage, mostly because the hand skill is concentrated there.
None of this makes the tea inherently better. It makes it earlier and more consistent.
A careful grower in Qiantang with the same clone and the same weather can beat a careless one in Xihu, and often does.
Skill is unevenly distributed, and it does not stop at the boundary.
What changes sharply at the line is land cost, and that passes straight through to the price of the leaf.
Harvest timing shifts too, because the core hills are picked first and the surrounding areas follow days behind.
A few days sounds like nothing, but for a green tea sold on the first flush it is the whole argument.
What the surround gives you
The surrounding areas have one real advantage, which is volume at a lower price.
Qiantang and Yuezhou produce the great majority of what is sold as Longjing, and much of it is made from the same Longjing 43（龙井43）clone as the core.
Machine firing is more common outside the core, and a machine presses a flatter, more uniform leaf than a hand does.
Uniformity is not automatically worse. It is just easier to recognise and easier to fake.
Further out still, provinces such as Sichuan and Guizhou make flat green tea in the Longjing style and sell it under their own names or none.
Some of this is excellent. What it is not is Longjing, and a seller who says otherwise is lying.
The style has travelled further than the name has, and flat green tea is now made well in several provinces.
Calling that leaf Longjing is fraud. Calling it flat green tea in the Longjing style is honest, and it should not cost a sale.
Paying for the difference
The honest test is whether you can taste the boundary, and most people cannot once the price gap passes three or four times.
Buy Xihu when you want the specific thing: pre-Qingming（清明）leaf from the core hills, bought early, from someone who can name the village.
Buy Qiantang when you want a daily Longjing and you are not serving anyone who will argue about it.
Skip the argument entirely when the tea is going into a pot at work with water from a boiler.
And be suspicious of any listing that says West Lake without a harvest date, because the core sells out early and the name is the single most borrowed label in Chinese tea.
A GI mark, a village and a date are what you are paying for. Without all three you are paying for a word.
One further question helps: ask what the raw leaf looked like, since the core hills pick small bud-and-leaf sets.
A seller who can describe the leaf before it was pressed is usually picking it rather than reselling it.
Where a listing offers both a core version and a surrounding one, buy both once and taste them side by side.
Plenty of people discover the cheaper one suits them, and that discovery is worth far more than any rule about boundaries.