On a standard computer keyboard, which key is used to temporarily change letters from lower case to upper case and to access the symbol characters shown above number keys?

Introduction / Context:
Typing on a computer keyboard often requires switching between lower case and upper case letters, as well as entering symbols such as !, @, # or %. Different keys have special roles in controlling how characters are entered. This question asks which key is used to temporarily change the character produced by a key, for example turning a lower case letter into an upper case one or a number into the symbol printed above it. Knowing this is basic computer literacy.

Given Data / Assumptions:

• We are using a standard QWERTY style keyboard layout.
• The question refers to temporarily changing the output of keys.
• Letters can be switched between lower and upper case.
• Number keys have secondary symbols printed above them.

Concept / Approach:
The Shift key is a modifier key that, when held down while pressing another key, changes the character produced. For alphabetic keys, holding Shift produces an upper case letter instead of a lower case one. For number keys on the top row, holding Shift produces the symbol shown above the number, such as ! above 1 or @ above 2. The Caps Lock key also affects case, but it toggles a permanent state and does not affect numeric symbols. Ctrl, Space bar and Backspace have completely different functions. Therefore, the key described in the question is the Shift key.

Step-by-Step Solution:

Verification / Alternative check:
If you look at your keyboard, you will see the word Shift printed on one or two keys. Experiment by typing the letter a alone (which gives a) and then holding Shift while pressing a (which gives A). Do the same with the number 1; by itself it produces 1, but with Shift it produces !. Caps Lock, on the other hand, stays on until pressed again and does not change numbers into symbols. Documentation for keyboard shortcuts also describes Shift as a modifier that changes characters or selects text. This real world behaviour confirms that Shift is the correct answer.

Why Other Options Are Wrong:

• Ctrl key: Used in combination with other keys to issue commands (such as Ctrl+C to copy), not primarily to change letter case or numbers to symbols.
• Space bar: Inserts a space character and does not modify other keys' output.
• Backspace key: Deletes characters to the left of the cursor; it does not alter the character produced by other keys.
• Caps Lock key: Changes letter keys to upper case when on, but it does not provide temporary change and does not affect number keys' symbols in the same way as Shift.

Common Pitfalls:
Some users confuse Shift with Caps Lock because both can produce capital letters. The key difference is that Shift is temporary and also affects the symbol characters on number keys, while Caps Lock is a toggle that affects only letters. Another mistake is to think Ctrl is used for case because many important keyboard shortcuts start with Ctrl. To answer questions correctly, remember that Shift modifies the next key pressed, especially for uppercase and symbols, whereas Caps Lock toggles letter case globally.

Final Answer:
The key that temporarily changes letters to upper case and numbers to their symbol characters is the Shift key.