Design patterns·The shape of the whole application
someone said three-tier and meant three folders, and someone else heard three machines.
Tier vs layer
Also calledtier versus layer, physical tier, logical layer
A layer is a logical grouping of code; a tier is a physical place it runs. Three layers can live in one process and one deployable, and usually should — the classic three-tier picture put them on separate machines because that was how you scaled in the nineties. Keeping the two words apart is what stops a layering discussion turning into a deployment argument.