AFTER

A folder becomes more useful when someone can tell what it contains without opening every file. Create an index.txt beside the collection. Start with the collection's name, purpose and the date you last updated the index. For each item, record its relative filename, a readable title and one sentence explaining what it contains. A relative filename such as notes/circle.txt describes a location inside the collection rather than a location that exists only on your computer. Keep the folder and its index together when you copy them. Add a few plain-language search words when the title is ambiguous. For a file called session-03.txt, "paper folding, square center, geometry" is more helpful than another unexplained number. This is a suggested convention, not a cataloging standard. Open the index in a basic text editor and use Find to locate a topic. Then follow the written path and open the named file. Repeat for a few items, including one inside a subfolder. Update the index whenever a file moves or changes meaning. An index describes the collection; it does not replace the files or prove that they remain readable.

A checksum is a compact value computed from data. SHA-256 produces a 256-bit digest, usually displayed as 64 hexadecimal characters. It can help you notice whether a saved file differs from the version for which you recorded the digest. Before copying a file, calculate its SHA-256 with a local checksum utility. Save the complete result together with the filename, the algorithm name and the date. After copying, calculate SHA-256 again from the destination file and compare every character. Use the same algorithm both times, and hash the actual file bytes rather than pasted text from an editor. A mismatch means the bytes or the recorded reference differ. Check that you selected the intended file before replacing anything. Editing a single character or changing line endings can change the digest even if a document looks similar on screen. A match is strong evidence of byte equality, not a statement about the document's accuracy, safety or creator. Someone able to replace both a file and its nearby checksum can make them agree. Keep a trusted earlier digest separately when you need a meaningful comparison over time.

Put a point on a sheet of paper. Call it the center. Choose a distance. Every point in the plane exactly that far from the center belongs to the same circle. A whole shape, held together by one rule. Hold one end of a string at the center and pull the other end taut with a pencil. Move the pencil around the center, keeping the center fixed and the string at the same effective length. The pencil traces an approximation of a circle. If the string stretches or your grip slips, the radius changes and the drawing wanders. The radius reaches from the center to the edge. The diameter runs from edge to edge through the center and measures two radii. A circle with a radius of four centimeters therefore has a diameter of eight centimeters. Check your drawing by measuring several center-to-edge distances. They should agree as closely as your materials allow. This is useful for a paper pattern or a drawing exercise; its precision comes from the construction, not from calling the result a circle.

Take a square of paper. Bring one corner onto the opposite corner, align the edges, and flatten the fold. Open the paper. The crease runs between the other two corners: it is one diagonal of the square. Now bring the other pair of opposite corners together. Open the paper again. The two creases cross at the center. Mark their crossing lightly if you want to use it for a paper design. Keep the sheet flat while checking the mark. Why does this work? Folding one corner onto another places the fold halfway between them. For a square, that fold also passes through the remaining corners because of the square's symmetry. Repeating the operation in the other direction locates the point shared by both diagonal symmetry lines. Start with a true square. A rectangle that is longer than it is wide will not align the same way when opposite corners meet, and an uneven cut can move the apparent center. The ideal geometry is exact; creases in real paper are only as accurate as the sheet and the folds. That difference is part of the experiment.

JSON stores structured data as text. Curly braces enclose an object made of name-and-value pairs. Square brackets enclose an array, which is an ordered list. Strings use double quotation marks. Numbers, true, false and null can also appear as values. Work on a copy of an export. A formatted view makes nesting easier to follow without changing the source file. If Python 3 is already installed, its documented command-line formatter can display a file with the command python3 -m json.tool archive.json. The filename is an example; the command reads it and prints a formatted representation. Nothing in this note runs automatically. If the parser reports an error, keep the original and investigate the reported location. A missing comma or quote may be the cause, but do not guess away damaged content. Successful parsing is only a first check. Python's default decoder accepts some extensions, including non-finite numbers, and keeps the last value when object names repeat. It does not establish that required fields exist or that the information is true. For an archive, also validate the expected fields, allowed values and duplicate policy.

A bookmark records where something was. A copy contains the material you saved. If the original address stops working, the bookmark may have nothing left to show. Begin by choosing the text, pictures or other material you want to retain, then save an actual file you can open locally. Give the copy a useful name and include the title, creator and date when you know them. If you save a web page, check which parts arrived: text may be present while images or other linked resources are missing. A screen that already loaded successfully is not proof that your saved file is complete. Keep another copy on separate storage, preferably in a different physical location. Open each copy to check it. A second filename on the same failing device does not provide the same protection as a separate device. Revisit the collection as devices and software change. Replace unreadable copies from a working one and move files to storage you can still use. None of these steps makes a file permanent. They make loss easier to notice and give you another place to recover from.

For a document whose main value is its words, keep a plain-text export alongside the original. Choose UTF-8 when the editor offers an encoding choice. It gives the text an explicit, widely documented character encoding instead of leaving the reader to guess. Include a title, creator, date and short description at the top. Use blank lines between paragraphs and descriptive headings. If the document refers to a picture, table or attachment, write what it is and where its saved copy belongs. A sentence that says only "see above" can become meaningless after export. Reopen the text in another editor and inspect accented letters, symbols and the start and end of the document. Compare important sections with the original. Plain text does not preserve all layout, images, formulas or interactive behavior, so retain the original when those features matter. This is a practical use of the Library of Congress's transparency and documentation principles. Simpler representation can make future interpretation easier, but a familiar extension alone guarantees nothing. The useful deliverable is readable content, known encoding and context that travels with the file.

A file can survive byte for byte and still be difficult to use if the required software or encoding is no longer understood. Test both possession and readability while a working original is still available. Choose a small, representative part of your backup. Copy it into a new, clearly named test folder. Do not restore over your working files. Open the restored documents with the software you expect to use later. Check the beginning and end, a few important details, and any attachments the documents require. For material intended to work offline, close the online version and try the saved copy without connectivity. Record which text and images remain available and which links or functions require a network. A test of three documents supports a claim about those three documents, not the whole backup. Write down the date, files, application and outcome. If something fails, preserve the failed copy and note the missing dependency before trying a different reader or making an access export. Keep the original too. This exercise can expose a present problem; it cannot guarantee that future hardware and software will behave the same way.

A CSV file represents a table as text. In a common form, commas separate fields and line breaks separate records. The first record may contain column names. A small example is a header reading item,count followed by a record reading pencil,4. A comma inside a value needs different treatment from a comma between values. In the convention documented by RFC 4180, surround a field containing a comma, quotation mark or line break with double quotation marks. A quotation mark inside a quoted field is written twice. That is why simply splitting every line at every comma can damage a table. Keep the original export untouched and inspect a copy in a text editor. Identify its separator, text encoding and whether it has a header. CSV implementations vary; do not assume every file uses the same conventions. Save a short description of what each column means, including units and date conventions. A value such as 03/04 is ambiguous without context. CSV can carry cell values, but the text alone does not explain the calculations or meaning behind them. Check representative records after importing into a different program.

You can write a nonnegative whole number using only 0 and 1. The trick is the place each symbol occupies. From right to left, the places are worth 1, 2, 4, 8, 16 and so on. Each place is worth twice its neighbor to the right. A 1 includes that place value in the total. A 0 leaves it out. Read 1010 as eight plus two: ten. Read 1111 as eight plus four plus two plus one: fifteen. The written digits have the same order as familiar decimal numbers, with the largest place value on the left. To write thirteen, take eight, leaving five. Take four, leaving one. Skip two and take one. The result is 1101. Check it by adding the selected place values back together. For a paper exercise, write the headings 16, 8, 4, 2, 1 and fill a row with zeros and ones. These five places represent every whole number from zero to thirty-one. Negative numbers and fractions need additional conventions; this exercise covers only unsigned whole numbers.

Imagine a dot traveling around a circle at a steady angular speed. Start it at the rightmost point and move counterclockwise. Watch its height above or below the horizontal line through the center. Plot that height against time and you trace a sine wave. Mark five moments, separated by quarter turns. The dot starts at the middle height, reaches the top, returns to the middle, reaches the bottom, then returns to its starting height. On your graph those heights are zero, plus the radius, zero, minus the radius, zero. Draw a smooth curve through them. The amplitude is the distance from the middle height to either extreme. The period is the time required for one complete repeat. If a turn takes four seconds, the period is four seconds; two turns in the same time halve the period. The dot never stops moving around the circle. At the top and bottom, only its vertical motion momentarily has zero speed. This graph is a mathematical model. Real recordings can combine many patterns and need not look like a single sine wave.

Make a triangle with side lengths three, four and five, all in the same unit. The angle between the three-unit and four-unit sides is a right angle. The five-unit side lies opposite it. The arithmetic is small enough to keep in your head: three squared plus four squared equals five squared. Nine plus sixteen equals twenty-five. In a right triangle, the squared lengths of the two shorter sides add to the squared length of the longest side. The converse also holds: if positive side lengths form a triangle and satisfy that relationship, the angle opposite the longest side is right. To try it on paper, mark two points five centimeters apart. Locate a third point three centimeters from one end and four from the other. Joining the three points gives the triangle. There are two possible third points, on opposite sides of the five-centimeter segment. Multiply all the lengths by the same positive number and the shape keeps its angles. Six, eight and ten works too. Measurements on paper are approximate, so treat this as a drawing and checking method, not a guarantee of construction accuracy.