Baseline Schedule
The original, approved project schedule established near the start of construction, kept unchanged afterward specifically so actual progress can be measured against it, the fixed reference point for "were we ahead of or behind where we planned to be."
Why it matters
A schedule that gets silently rewritten every time the project falls behind stops being useful as a way to measure performance: there's no "behind" if the goalposts keep moving. The baseline schedule is what lets everyone agree on how much of a delay is real and who's responsible for it.
On a real project
A project's baseline schedule shows framing complete by week 12. When framing actually finishes in week 15, that three-week gap against the baseline becomes the starting point for figuring out what caused the delay and whether it qualifies for a schedule extension.
Who this matters most to
A Scheduler establishes the baseline schedule early in the project and formally tracks the working schedule against it every reporting period, rather than replacing it. A Project Manager relies on the baseline as the reference point in any dispute over who caused a delay, since it's the agreed-upon plan both sides signed off on.
Where this goes wrong
A scheduler quietly resets the baseline schedule every time the project falls behind, rather than tracking variance against the original plan. By the end of the project, there's no clear record of how much total delay occurred or when it happened, which becomes a serious problem when the owner disputes a request for additional time and money.