Crude is one market with a handful of benchmarks that rarely drift more than a few dollars apart, because a tanker can take a barrel almost anywhere. Natural gas was, until recently, several regional markets that barely touched, because gas moves by pipeline and a pipeline ends where it ends. The US benchmark is Henry Hub, a pipeline interconnect in Erath, Louisiana, and the delivery point for the NYMEX NG futures contract, quoted in dollars per million British thermal units (MMBtu). The European benchmark is TTF, the Title Transfer Facility, a virtual trading point on the Dutch grid, quoted in euros per megawatt-hour.
Liquefied natural gas is what connects them. A US liquefaction plant buys pipeline gas priced off Henry Hub, cools it to a liquid, and ships it across the Atlantic or Pacific. Many US export contracts are priced as a multiple of Henry Hub plus a fixed liquefaction fee, so the cost of a delivered cargo rises with Henry Hub, and the value of that cargo in Europe is set by TTF. The spread between the two therefore describes how open the transatlantic trade is.
The spread does not close to zero, and it should not. Liquefaction, shipping and regasification cost several dollars per MMBtu between them, so a wide spread is the normal state, not an arbitrage waiting to be taken. What moves the spread is capacity: US export terminals run close to full most of the time, so when Europe bids up TTF there is little extra US gas that can answer, and the gap widens rather than closes. When a terminal trips offline, Henry Hub loses a buyer and falls against TTF on the same day.
The US contract has its own calendar. NG trades on a monthly strip, and trading in each contract ends on the third-last business day of the month before delivery, around a week later in the month than WTI futures, which expire near the 20th. Desks quote it as strips: the Nov-Mar winter, the Apr-Oct summer, and spreads between them, because the seasonal shape is the dominant feature of the curve.