When adventurers travel into other planes of existence, they are undertaking a legendary journey across the thresholds of existence. Such a journey is often the stuff of legend. Braving the realms of the dead, seeking out the celestial servants of a deity, or bargaining with an efreeti in its home city will be the subject of song and story for centuries to come.
Travel to the planes beyond the Material Plane can accomplished in only two ways: being summoned by a spell, or by using a gate. Both methods are essentially the same. Effects which allow you to enter the Astral Plane or Ethereal Plane from the Material Plane are still functional.
Gates
Gates are stationary and permanent inter-planar connections that allow travel between a specific location on one plane and a specific location on another plane. They are always "open," allowing people to enter them freely from either side. This has created the necessity for some places to have constructs designed specifically to operate and defend planar gates at all times.
Gates can only allow travel between planes on the same level: Gates on the Prime Planes only allow travel between the Prime Planes, while Gates on the Inner Planes only allow travel between the Inner Planes, and so on. It is not possible, for instance, for a Gate to be opened from the Material Plane to the Plane of Delight.
While Gates once upon a time existed that led to planes on other levels, they no longer operate.
Summoning
There are spells and rituals that can be used to summon creatures from other planes. Summoning any creature requires the summoner to set a condition that must be met before the condition can end. Some figures from history purposefully made these conditions extremely difficult, or outright impossible, so that the creature they summoned could never return.
No matter whether you are using a Gate or are Summoned, all you are doing is creating an entry and exit door to your destination. You must still traverse one of the Transitive Planes in order to actually arrive safely at your destination.
