Portals and Gates (PWH)

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Originally (a long time ago), the term "portal" referred only to a doorway leading to and from Sigil itself. This distinguished portals from gates, which were any magical links between two areas. If one end of the link wasn't anchored in the Cage, then it wasn't really a portal, it was a gate. Nowadays, "gate" and "portal" are virtually interchangeable terms. A few people stick with the strict original definitions, but most see no reason to do so. The portals of Sigil work in the same way as gates everywhere else. Planewalkers rarely make the distinction and use both words, except the doorways are always called portals when found in Sigil and always called gates when found in the gate-towns of the Outlands. 
Another point of disagreement among planars revolves around where a portal can be found. Some sources say they're found only in doorways, at least in Sigil, but bloods know that they can exist in any archlike opening or bounded space. Examples include holes, windows, arched roof supports, sewer grates, barrel mouths, and hatches. When activated, portals re-main open for only a limited time (except for the permanently open gates on the Great Road and a few other paths, as noted below). Also—again with a few notable exceptions—portals and gates always require a magical key to activate them. So here's as good a definition of a portal or gate as a body's going to get: a doorway that must exist within a bounded space, is open for only a limited time, and requires a key. Other methods can take a planewalker from one plane to another, but they're not technically gates or portals. (They might still be called gates by those who aren't hung up on nitpicky terminology, though.)
Portal travel has two tricks: finding the portal and determining its key. No, make that three tricks (as per the Rule of Threes, of course)—the third is determining what's on the other side and figuring out how to survive there.
==Finding & Opening a portal==
Portals are usually found in a fairly distinct arch or doorway, especially in Sigil. The arch can be hard to find, though; sometimes the branches of two trees form an archlike aperture that can be activated as a portal. Or the portal might be a cave mouth, the underside of a bridge, or between a statue's legs. At least portals are usually man-sized, so a body doesn't have to worry about squeezing through. Most portals are also nonmobile, although plenty of exceptions exist (barrels, wardrobes, chests).
Planars can detect portals naturally on a roll of 1, 2 or 3 on 1d6 when looking, or on a roll of 1 on 1d6 if casually observing. This ability doesn't grant the knowledge of gate keys or where a portal goes. Magical true sight or warp sense also reveal a portal's existence (warp sense can determine the key, as well). A body can only see portals that're actually there, though. Sounds like an addle-coved thing to have to say, right? Well, think on it a minute—portals can shift and fade, and if this happens, the portal isn't there anymore. A portal's either there or it ain't, and if it's there, it can be detected. If not, it can't. An active portal is one that's been opened by a key (rather than just being there), but a portal doesn't need to be open to be detected.
Once a portal has been found, a planewalker still has to discover its key and (unless she's a real addle-cove) where it leads. To learn the right key, a hasher's got to use warp sense or she's got to know who to ask. SpelJs like legend lore and contact other plane can help reveal a gate's destination as well.
A gate key isn't always obvious, either. Usually it's an object, like a bone, a flower, a spoon, or a boot. Sometimes the key is a singular object, such as a specific dagger. OccasionalJy it's one of a general group of things, such as any wine glass. At still other times, a gate key is a far more esoteric thing, such as a word, a gesture, a musical note, or an emotion. Gate keys often relate to, or at least suggest, the destination plane of the portal. It's not always that easy, though.
Activating a portal with a key isn't an exact science. Usually, a body simply has the key in her possession, and walks through the gate. Sometimes a key must be physically used in some way, such as holding it forth with a particular attitude or touching it to the gate a required number of times. Obviously, if the key's a word, sound, action, or thought, it must be performed accordingly. A basher doesn't even have to want to activate a portal. If she's got the key and uses it (inadvertently or not), the portal opens. Sometimes the berk with the key doesn't even have to be the one who goes through the portal, although most of the time (especially when the key's an object) the key must be carried through the portal by the first person going through—that's the action that activates the gate in the first place.
Once a portal's been activated by the key, it crackles with energy or some other visible effect that anyone, prime or planar, can see. Further, if a body looks closely, the destination can be seen through this open gate, at least as a faint outline. As stated above, the portal stays open for just a few sec-onds, usually long enough for six (or so) individuals to pass through it. If a berk hesitates, he might not make it through. It's also a possibility that he might get caught passing through as the gate closes. What happens then? He gets thrown one way or another out of the portal—a body can't exist in between. There's a 50/50 chance of being tossed to the portal's destination or back to the starting point. As a barmy Cager game, some bubbers activate portals and jump in just as they close, wagering on whether they'll go through or not. 
A portal's always in an enclosed space, like a doorway, right? Well, every doorway has two ways to cross through it—that's what makes it a doorway. Some activated gates can be entered from either side. If a tavern's doorway in Sigil holds a portal to Acheron and a leatherhead with the key activates the portal from outside the tavern, some bubber stepping out of the tavern through the doorway at the same time might find himself on Acheron (poor sod). A body needs to take a modicum of care when he activates a portal—not a few addle-coves have ended up on planes they didn't want to be on because they didn't use a gate correctly or carefully (or they didn't know they were using a gate at all).
Finally, note that spells such as hold portal, knock, wizard lock, and the like have no effect on portals at all! Only spells created specifically to deal with planar portals—such as gate ward, surelock, and warp sense—have any effect on portals. 
==Portal Types and Destinations==
To further complicate matters, it's a fact that all portals aren't permanent. Some temporarily appear or disappear. Some change their keys. Shifting portals move one (or both) ends to different locations at different times, either following a set pattern or simply changing randomly. Many bashers make their living by mapping the patterns of shifting portals and seHing the logs to planewalkers, but some just have no pattern at all. 
Portals can be one-way or two-way. One-way portals offer travel in one direction only, so a body going through a one-way portal to any given plane has to find another way back. Two-way portals provide transportation to and from a given point, and often require a different key for each way of egress.
In Sigil, most portals are two-way, and many require the same key both ways. The Lady of Pain has complete control over the portals in the Cage, closing or opening them as she desires. Further, she prohibits any other means of leaving or entering Sigil. Sometimes she creates new portals where none existed before, and changes the destination point of others at her whim. Whether powers out on the planes can do this to gates as well is unknown. It is clear that no power can enter Sigil by any means—the Lady keeps 'em out. 
The portals on the Great Road, the World Ash, and Mount Olympus don't need keys to open them. They are, in a sense, always active. The gates on the Great Road, due to their usefulness, become centers of commerce and sometimes military strategy. Towns and military camps often form near or around them. Occasionally some local leatherheaded lord claims to own one of the gates and sets up guardians or even walls around it. 
==Specific Portal Dangers==
If a body finds a gate to the Astral Plane, he should expect trouble on the other side. Portals leading to the Astral have a way of grounding themselves in or near githyanki fortresses. In fact, portals generally tend to ground in solid, often occupied areas, and civilization springs up near them by their very nature. This doesn't mean that all portals move—bar that. It just means that if a portal leads to a particular plane, the specific site of arrival's usually in a civilized area with the most stable terrain possible. 
Due to Limbo's fluctuating nature, portals to that plane are rare and their locations constantly change as the archways supporting them morph, change, or disappear altogether. Established gates on the Lower Planes often serve as staging grounds or common battle sites for the Blood War. A wandering berk can accidently find himself in a yugoloth camp by stumbling through a portal on the dark portions of the Great Road. 
Planewalkers using one-way portals always run the risk of stranding themselves on the destination plane. Canny cutters know that t~is is particularly true for one-way portals to the Prime. Some prime-material worlds don't have many portals. A few practically have none. A planewalker finding himself hipped in such a locale ... won't be a planewalker any longer.
==Portal Tricks==
Naturally, planewalkers have learned a few tricks to bypass or take advantage of some of the restrictions related to portal use. 
Peeking through: Normally, folks can't simply stick their heads through a portal and pull it back to see what's on the other side. Once a body puts part of herself into a portal, she's committed and can't pull anything back through. Why? Well, lots of portals only function one-way. They allow a berk to pass through, but not return. Further, many two-way portals require separate keys depending on which direction a traveler passes through the gate. 
But if a planewalker has both required keys for a two-way portal (or if only one key is needed for both ways), then it's possible for her to peek through (by sticking her face through enough so that her eyes appear on the other side), stick a finger in to test the temperature, or otherwise put part of herself into the gate and bring it back. 
Pushing someone through: A sneaky way to get rid of an enemy or a troublesome berk: Subtly plant a gate key for a one-way portal on the rube and push or trick him through the portal. He can't return unless he's got the ability to travel the planes on his own. This is sometimes known as "peeling someone through a gate," or even "hipping a rube." 
Sneaking through: When a portal activates, it stays open for a few seconds. The amount of time varies for each portal, but it's never shorter than one or two seconds and never longer than 10 (unless it's permanently open, or an otherwise exceptional portal—every rule gets broken, as all planewalkers know). Still, that's usually enough time for a quick spiv to leap through the portal. This means that a body can wait near a portal and dive through when someone else brings the proper key. Invisibility works well here, so that the cutter can slip through unnoticed. Otherwise, the sneak has to deal with those he traveled with (uninvited). 
If a portal is two-way, and the same key activates both directions of egress, a body can even wait on one side until a bunch of berks come through from the other side and then slip past them, assuming he doesn't bump heads with someone as they cross over. 
==A Few Exceptions==
If a body searches long and hard enough, he'll certainly find exceptions to the general rules defining portals as explained above. Portals without keys, not enclosed in a bounded space, or permanently open (or all of the above!) aren't outside the realm of possibility—after all, it's an infinite multiverse, full of infinite possibilities. But these exceptions are certainly far rarer and probably more dangerous than a cutter wants to bother with. 
For a closer look at a specific exception, curious planewalkers can jaunt onto the prime-material world of Toril. As noted previously, Toril holds a great number of keyless gates leading to various planes. These artificially created portals were constructed using a high-up mage spell called worldwalk and some kind of permanency magic—which, by all rights, should be impossible. After all, mortals can't create permanent gates, right? And if they can, then why doesn't some enterprising blood set himself up in the portal-making business? tals just can't be recreated. It's a peculiarly Prime-specific spell, or maybe even a Toril-specific spell (not an unreasonable assumption, as the power that oversees magic in that crystal sphere is a particularly generous one). Planewalkers are simply stuck with the inconveniences of portals and keys, or with the other traveling methods described below. 
Simply put, it doesn't work—not out on the planes, anyway. Somehow, the process that produces those Torilian portals just can't be recreated. It's a peculiarly Prime-specific spell, or maybe even a Toril-specific spell (not an unreasonable assumption, as the power that oversees magic in that crystal sphere is a particularly generous one). Planewalkers are simply stuck with the inconveniences of portals and keys, or with the other traveling methods described below.