What does it actually mean?

What is that "YKK" that you see on your zippers? 10 codes explained

Have you noticed "YKK" stamped on your zippers or that "RSVP" on an invitation? What does the mysterious "SSSS" on your boarding pass mean? These little letters and codes are everywhere, but if you stop and think about it, there are plenty whose meaning we don’t really know. Behind many of them are surprising stories, foreign languages, old technologies, and even a few myths. Here are 10 everyday codes and abbreviations you’ve probably seen countless times without knowing what they actually mean.

You’ve probably noticed that almost every zipper on your clothes has the letters "YKK" stamped on it. But have you ever wondered what those three letters actually mean?

They stand for Yoshida Kogyo Kabushikikaisha, the name of the huge Japanese manufacturing company that produces roughly half of the world’s zippers. Founded by Tadao Yoshida in Tokyo in 1934, the company became a global zipper powerhouse, and its familiar YKK logo now appears on the pull tabs of zippers around the whole world.

Every time you receive an invitation to a wedding, birthday, or other important event, there it is: "RSVP" in all caps. But what do those four letters actually mean?

The initialism comes from the French phrase Répondez s'il vous plaît, meaning "please reply," or literally, "respond, if it pleases you." The request for a response originated in Europe and was adopted by the American elite, who embraced French court etiquette in the 18th century, preserving the French expression. Today, RSVP has become a practical yet elegant way to ask guests to confirm whether they’ll attend, helping hosts know how many people to expect.

One of the most famous codes in the world, SOS is the quintessential emergency signal, recognized across languages and around the globe. But what do those three letters actually mean?

Contrary to popular belief, SOS doesn’t stand for "Save Our Souls" or "Save Our Ship." That misconception took hold in the early 1910s and became widespread after the sinking of the Titanic in 1912. In reality, the letters don't stand for any words; the signal was chosen internationally at the Berlin Radio-Telegraphic Conference in 1906 and went into effect in 1908 because of its distinctive Morse code sequence. Three dots, three dashes, and three dots (... --- ...) create a simple rhythm that is easy to send and recognize during an emergency.

Have you ever noticed a strange "SSSS" code on your boarding pass? Perhaps you saw it the same day you were pulled aside for an extra security check before boarding. That’s no coincidence.

It stands for "Secondary Security Screening Selection". It’s a designation placed on some boarding passes by security authorities, such as the TSA, indicating that the passenger has been selected for additional screening, which can include a pat-down, baggage inspection, or explosive-residue swab. But seeing SSSS doesn’t necessarily mean there’s anything suspicious. Passengers can be selected based on certain travel or booking criteria, but they may also be chosen at random. If you see SSSS on your boarding pass, give yourself an extra 30 to 45 minutes, since the additional screening can take more time.

In countless history books, timelines, and educational materials, we see the abbreviations B.C. and A.D., or their modern alternatives, B.C.E. and C.E. But why do we use them, and why are there two versions?

We use these abbreviations to date historical events, counting years backward before the traditional birth of Jesus and forward after it. B.C. means "Before Christ," while A.D. stands for Anno Domini, a Latin phrase meaning "in the year of our Lord." The modern alternatives use more secular language: B.C.E. means "Before the Common Era," and C.E. means "Common Era." Both systems use the same year numbers and the same calendar points; the difference is simply whether the terminology has an explicitly Christian reference.

At the end of a U.S. mailing address, right after the city and state, we have the familiar ZIP Code. We’ve been using it for decades, but many people still don’t know exactly what those three letters stand for.

"ZIP" stands for "Zoning Improvement Plan." The U.S. Postal Service introduced the system in 1963 as mail volumes continued to rise, making faster, more efficient processing more and more important. These codes divided the country into geographic areas, giving postal workers a more organized way to sort and deliver mail. They also became an important part of the Postal Service’s growing use of automated sorting machines, helping mail literally "zip" through the system.

Have you noticed that whenever you forward an email, the abbreviations CC and BCC are right there? These familiar letters actually come from a time long before email.

CC stands for "Carbon Copy" and means other recipients can receive a copy while their addresses remain visible. BCC, or "Blind Carbon Copy," also sends a copy, but hides the recipient’s address from everyone else. The terms date back to the days of typewriters and carbon paper, which allowed people to make duplicate copies of a letter as they typed. A "CC" notation showed who else received a copy, while a "BCC", or blind copy, kept that additional recipient in secret.

"P.S." is undoubtedly one of the most familiar abbreviations used in letters and emails around the world. But do you know exactly what it means and why we use it?

Before computers and smartphones made it easy to edit text, writing a letter by hand meant that forgetting something often meant adding it afterward. "P.S." comes from the Latin post scriptum, meaning "written after." Adding it at the bottom of a letter gave you a simple way to include an extra thought or piece of information without rewriting the entire page.

You’ve probably used your debit card PIN countless times. Most people know theirs by heart, but how many of us actually know what those three letters stand for?

"PIN" stands for Personal Identification Number, a security code used to verify that the person using a card or accessing an account is authorized to do so. It dates back to 1966, when the PIN system was developed as part of efforts to create an automated cash dispenser. The following year, in 1967, the first ATM was introduced for public use. But there’s one small mistake many of us make without realizing it: "PIN number" is redundant, because the "N" already stands for "Number."

Whenever we read an explanation or definition, we often come across "e.g." and "i.e.", especially in dictionaries and reference books. They may look similar, but what do those letters actually stand for?

"E.g." is short for the Latin exempli gratia, meaning "for example." On the other hand, "i.e." comes from the Latin id est, meaning "that is" or "in other words." The two are easy to confuse, but they serve different purposes. Use "e.g." when you want to give a few examples of something, such as "I like tropical fruits, e.g., mangoes and pineapples." And "i.e." when you want to clarify or restate exactly what you mean, such as "I am a vegan, i.e., I do not eat any animal-based products."

Odd words, old meanings

These 10 idioms and phrases are still in use, but don’t always make sense

How much is a notch when it comes to volume? Who is Bob, and why is he everyone’s uncle? What is the difference between a spider web and a cobweb? Languages change with time and use, and English is no exception, but some phrases remain frozen in time, and while we still use them, we don’t always know their literal meaning. Here are 10 idioms and words that don’t make sense, but once did.

We often use the phrase "to a T" (sometimes spelled "to a tee") to indicate something done perfectly, with precision. But is it about the letter "T"? Not really. The original phrase used to be "to a tittle," with tittle being the proper name for the dot of the "i" or the "j," and it conveyed the idea of doing something accurately, down to the smallest detail. Eventually, "tittle" was shortened to "T," but the phrase had already stuck, even if its literal meaning was lost.

Although "fathom" might sound very formal, we all know that it is synonymous with "understand." But did you know that fathom is actually a nautical unit of measurement? From the Old English fæðm ("the span of the arms" or "embrace"), it represented the distance between the left and right fingertips of outstretched arms. Eventually, a "fathom" was standardized to about six feet or two yards, and was mainly used by sailors to measure water depth. It was this connection with depth that led the word to take on a figurative meaning of "getting to the bottom" of something, or understanding it thoroughly.

Popularly, we understand that a "red herring" is something meant to give false clues or distract from another pressing issue. But why a herring? Herrings turn red when smoked and also acquire a strong, pungent smell, which made them a great tool for training hunting dogs, as the smell of the fish covered the trail of the prey. As a metaphor, though, it was popularized in the early 19th century by journalist William Cobbett, who used it to describe how the press was easily swayed by false news.

If something is a "rule of thumb," it means that it’s a common guideline, usually based on experience rather than science. In its origins, this phrase was quite literal: thumbs served as a rule. Tradesmen often used the width and length of their thumbs to take and remember measurements, a system that, though practical and effective, only worked for the owner of the thumb in question. Its use as an idiom comes from 17th century writers comparing this popular method with more scientific measuring systems.

If someone asks us to "take it down a notch," how much should we lower our volume? With "notch," this idiom refers to mechanical dials, like the ones used in a radio, where the notches signal the different levels. And while radios only date back to the 20th century, the roots of this idiom go all the way back to the 17th century and to the phrase "take it down a peg," related to wooden pins used for measurements.

Is there a difference between a cobweb and a spiderweb? Although they are synonymous, most dictionaries will make the distinction that "cobweb" is particularly used for dusty, abandoned spiderwebs. But what does the "cob" part mean? "Cobweb" is derived from the Middle English coppeweb, composed of coppe ("spider") and web ("woven fabric"). If we go one step further, coppe comes from the Old English attorcoppe ("spider"), a mix of attor ("poison") and coppe ("head" or "round thing"). So, despite the slight difference in their actual use, "cobweb" and "spiderweb" mean the same thing.

It isn’t difficult to see how "breaking the ice" might have originated from people actually breaking ice, but what many ignore is that this custom (and the phrase) is rooted in diplomacy and trade. When large trade vessels were unable to advance due to frozen rivers, canals, or harbors, it was common to send people in a small boat to "break the ice" and allow the ship to continue its course. In some places, however, locals would send "icebreakers" to clear the path for a foreign vessel as a welcome and a sign of goodwill. The metaphorical use of the phrase exists in many languages, but it was first recorded in English in the 16th century.

The idea of "letting down your hair" is popularly associated with relaxing and letting formalities aside, but the phrase is rooted in an actual Medieval and Renaissance custom. Traditionally, single women would keep their hair loose and uncovered, but would pin it up once they married. It was not proper for a married woman to be seen in public with their hair down, something they would only do in the comfort of their home, foregoing elaborate hairstyles and uncomfortable pins. Eventually, loose hair lost its social status meaning, but the connection with relaxation remained.

According to historians, no one knows who Uncle Bob is, or what his connection is with simple and easily accomplished tasks, but popular folklore claims to have the answer. According to this theory, the phrase refers to British Prime Minister Robert Gascoyne-Cecil (who went by Bob), appointing his nephew Arthur Balfour as Chief Secretary for Ireland, a position he was woefully unqualified for, but that was made possible because "Bob’s your uncle".

Have you ever wondered why midwives have that name? After all, there is nothing "mid" nor necessarily "wifely" in helping someone give birth. The answer takes us back to Old English, where mid meant "with" or "together with," and wif (the origin of "wife") meant "woman," "female," or "lady," and not necessarily a married female. First recorded around the year 1300, "midwife" was an accompanying person (typically a woman) who assisted mothers during labor. Although the literal meaning of the term is lost to us, the importance of the role was surely the reason this word survived through the centuries.