You’ve built the model. Named inputs, calculated columns, a checks row, the works. Then you share it — and within a week someone has typed 4200 over a formula, dragged a Table sideways, and “tidied” the inputs sheet into abstract art. Nobody meant harm. Spreadsheets are just open fields, and colleagues wander.

Sheet protection builds the fences: which cells accept hands, which don’t. Set up right, it converts “please only type in the yellow cells” from an email nobody reads into a physical property of the file.

The two-step that everyone gets backwards

The confusing part is that protection is two settings, not one — and the order matters:

  1. Unlock the input cells first. Every cell is born with a Locked flag, on by default (Ctrl+1 → Protection). The flag does nothing — until protection turns on, at which point every still-locked cell freezes. So: select the cells people SHOULD edit — the inputs, the entry columns — and untick Locked.
  2. Then protect the sheet. Review → Protect Sheet. Now the locked majority is read-only, and the unlocked minority still takes typing. Tab even walks between unlocked cells like a form.

Backwards — protecting first, wondering why nothing is editable — is everyone’s first attempt. Unlock, then lock the world.

protected sheet — locked grey, unlocked green =SUM(...) 🔒 =XLOOKUP.. 🔒 rate: 4.5% =IF(check.. 🔒 units: 120 =PMT(...) 🔒 "4200" ✗ "135" ✓ lands here typing bounces off the machinery and lands in the fields — the please-only-edit-the-inputs email, enforced by the file itself
Locked cells shrug typing off; unlocked cells accept it. The fence does what no amount of cell-colouring convention ever did.

Three refinements worth their clicks: the Protect dialog’s checkboxes let you permit safe actions on locked sheets (sorting, filtering, pivot use) — tick what the workflow needs rather than freezing everything; Hidden (Locked’s neighbour in Ctrl+1) hides a cell’s formula from the bar while showing its value — for the rare genuinely sensitive logic; and Protect Workbook (the button beside it) fences the structure — no deleting, renaming or un-hiding of sheets — which is what actually stops the classic “deleted the Calc tab” incident.

Pair the fences with the course’s other guard rails and entry becomes genuinely hard to fumble: validation policing what goes in the unlocked cells, protection policing where — and upstream of both, the Forms question: should this typing be happening in a workbook at all?

What protection is not

Say it plainly, because the stakes deserve it: sheet protection is a fence, not a safe. The password is trivially bypassed by anyone motivated — it exists to prevent accidents, not to defeat adversaries. Confidential data is protected by where the file lives and who it’s shared with — permissions, not passwords-on-sheets. And never let a protection password become the only copy of itself: an un-openable model is the inheritance problem at its worst. Record it where your successor will look.

The mindset shift, final form: a shared workbook is an interface, and interfaces get designed — inputs open, engine fenced, structure pinned, checks watching. People wander; good fences mean it no longer matters. That’s not distrust — it’s hospitality. The yellow cells are finally, enforceably, yellow.