Groups & Subtotals

Grouping lets you collapse detail you don't need to see right now. Select rows (or columns) and choose Data ▸ Group. A bracket with a minus button appears beside them — click it to collapse the group to a plus, click again to expand.

Groups can be nested: months inside quarters inside years.

Unlike hiding, grouping is obvious to anyone who opens the sheet, so detail is never "lost" — which makes it the better choice for reports.