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There is no equals key, and there are no parentheses. You give the calculator the numbers first and the operation last.

To work out (3 + 4) × 5: press 3 ENTER 4 +, which shows 7, then 5 ×, which shows 35. ENTER is what separates one number from the next; the operation keys act on the two numbers already there.

The upper line of the display is Y and the lower line is X — the two numbers an operation will use. Watching those two lines while you type is the fastest way to build the habit.

While you are still typing, the backspace key removes the last digit. Once the number is on the stack, backspace clears X entirely.

If the mistake has already gone through an operation, use undo — fifty steps of it, covering the stack, the registers and any part-typed number. LAST x is the other rescue: it brings back the argument the last function consumed.

The stack — X, Y, Z and T — starts empty each time the app launches, in the way a calculator that has been switched off does.

Your ten numbered storage registers, the paper tape, and every setting (angle mode, display format, decimals, separators, haptics) are kept between launches. If you want a value to survive, store it in a register with STO.

Open Settings — the gear key on the bottom row of the keypad, or ⌘, on the Mac — and choose DEG, RAD or GRAD under Angle Mode. The current mode is shown as a small annunciator above the display, so you can always see which one is active.

On the Mac the same three choices are in the Mode menu.

They are the shifted functions. Press the orange f key and the keypad switches to the orange legends for one keystroke — so f then the SIN key gives you arcsine, and so on. Pressing f a second time cancels it.

Functions that do not fit on the keypad — hyperbolics, combinations and permutations, coordinate conversion, hours-minutes-seconds — live in More Functions, reached from the MORE key in the top row.

The number keys, ., + - * /, Return for ENTER, ⌫ for backspace and Esc to clear X all work as printed. Letters follow the key faces where they can: s sine, c cosine, t tangent, l natural log, g common log, q square root, i reciprocal, p π, n change sign, e exponent entry, x swap X and Y, r roll down, m store, v recall.

With the Command key: ⌘Z undo, ⇧⌘Z redo, ⌘C copy X, ⌘V paste, ⌘E swap X and Y, ⌘D roll down, ⌘U roll up, ⌘L LAST x, ⌘K clear X, ⇧⌘K clear the stack, ⌘T paper tape, ⌘Y convert units, ⌘B constants, ⌘M more functions.

On the Mac, ⌘C copies X and ⌘V pastes a number into it. The paper tape has a Copy All button that puts the whole session on the clipboard as plain text.

Within the app, tapping any line of the paper tape that carries a number lifts it straight back onto the stack.

Yes — Settings, under Keyboard, has a Haptic Feedback switch. It is an iPhone setting; the Mac version has nothing to vibrate.

The app runs on iPad today in iPhone compatibility mode, which works but does not make good use of the screen. A dedicated iPad layout is planned as a free update.

Free: the four-level stack, arithmetic, square roots and squares, reciprocals, percentages, the ten storage registers, undo, and the everyday keys such as integer and fractional part, absolute value, modulo and factorial.

Enter Pro is a one-time purchase with no subscription. It adds the scientific functions (trigonometry, logarithms, exponentials and powers), the financial keyboard with the HP-12C's function set, the paper tape, unit conversions, constants and the extended function list. Keys that need it carry a small lock until it is bought; tapping one opens the purchase. Anyone who bought Enter before it went free has Pro already.

Above the gear key sits an orange FIN legend. Press f and then the gear, or touch and hold the gear, and the keypad takes on the HP-12C's function set: n, i, PV, PMT and FV across one row, with the cash-flow keys in orange above them, and the bond, depreciation, date and statistics functions on the rows around them. The same legend, now reading SCI, brings the scientific keyboard back. The choice is also in Settings, and in the Mode menu on the Mac.

The five financial keys work the way HP's financial calculators have them: after a number they store it, and pressed straight after another financial key they solve for their own value. Touch and hold any of the five to see what it is holding.

No. The app is free on both, and Enter Pro is one purchase that unlocks both: buy it on the iPhone and the Mac picks it up through your Apple Account, or the other way round. If a device has not noticed yet, Restore Purchases in Settings fetches it.

There is no subscription.

Refunds are handled by Apple rather than by me — Apple is the seller of record for everything on the App Store. Request one at reportaproblem.apple.com.

If something about the app is not working, do write first. It is usually faster to fix than to refund.

None. There is no analytics, no account, and no network code of any kind — your registers, tape and settings are stored on your own device and nowhere else. The full details are on the privacy policy page.