Two lifters both squat three times a week for four weeks. One of them keeps the same rep range every session and adds weight or reps each week. The other changes the rep range from session to session and repeats a similar pattern week to week. Both are periodizing. They are just periodizing on different clocks, and the difference only gets clear once you look at an actual week instead of a diagram.

Linear periodization: the clock is the week

Linear periodization moves the difficulty dial once per week, in one direction. A common shape: start with higher reps and lower relative load, then step toward lower reps and heavier load as the weeks pass. Monday's squat in week 1 might be 4 sets of 8 at a moderate weight. Monday's squat in week 4 is 4 sets of 4 at a heavier one, and every session that week looks roughly the same as the others in it. What changes is which week you're in, not which day.

The appeal is obvious: it's easy to follow and easy to write. You know exactly what next Monday looks like the day you write week 1. The tradeoff is that it commits to one direction for the whole block, so if week 3's heavier target turns out to be too much, there is no built-in day within that week that backs off. Only the deload at the end of the block absorbs it.

Undulating periodization: the clock is the day

Daily undulating periodization (DUP) moves the same dial, but resets it every session instead of every week. A three-day squat week might run 4 sets of 10 on Monday, 5 sets of 5 on Wednesday, and 3 sets of 3 on Friday, then repeat something close to that same pattern the following week instead of pushing every number up. The volume-heavy day and the load-heavy day both happen inside the same week, every week, rather than sitting in different phases months apart.

The variety is the point: a heavy day doesn't also have to carry the fatigue of being a high-volume day, because the high-volume work happened two days earlier on a different day. It asks more of the plan, though. Three different rep ranges for the same lift in the same week means three different weights to calculate and three different recoveries to manage, which is a lot to hold in your head without something tracking it for you.

The same lift, two different weeks

Squat, three sessions, same starting point:

Linear, week 2 of 4: Monday 4x6 at a load with a couple of reps left in the tank. Wednesday, same numbers. Friday, same numbers. Week 3 pushes every session to 4x5, heavier.
Undulating, any week: Monday 4x8 at a lighter load. Wednesday 5x4 at a heavier one. Friday 3x2 near the top of what's comfortable that week. The following week looks close to the same shape, not heavier across the board.

Written out that way, the two approaches aren't really competing philosophies about squatting. They're two different answers to one question: when does the plan change, week to week or day to day.

Which one is "better" is the wrong question

Neither approach fixes a program that has bigger problems, like drifting away from the progressive overload that actually adds up over months, or skipping the deload that lets a hard block's fatigue drain before the next one starts. Linear periodization is simpler to run by hand and suits someone peaking one lift for a specific date. Undulating periodization suits someone who wants variety inside every week, or who trains a lift often enough that hitting the same rep range three times in seven days gets stale. Plenty of real programs blend both: undulating shape within a week, with that whole pattern nudged slightly heavier block to block, which is linear progression riding on top of daily variation.

The honest answer is that head-to-head comparisons between the two are mixed, and the practical difference for most lifters is smaller than either camp's advocates suggest. What actually predicts whether a program works is whether you can follow it consistently and whether it includes real overload and real recovery. A DUP plan you skip half of loses to a linear plan you run for three months straight.

How this shows up in a real plan

Where this stops being theoretical is in how the plan gets built, because "linear" and "undulating" are just different ways of setting targets across weeks and days, not different apps or different systems. Zirv's plans are built as weeks made of days made of exercises and sets, so a plan can carry a different rep and load target for each day of the week and repeat that same shape every week, which is undulating, or hold the same target across every day of a week and step the numbers up week to week, which is linear, inside the exact same plan structure. Either way, the app tracks what you actually lifted against what the day called for and carries that into the next session's target, whichever clock the plan is built around.

The takeaway

Linear periodization changes the plan once a week, in one direction, and repeats each week's numbers across every session in it. Undulating periodization changes the plan every session and repeats a similar shape from week to week instead. Neither is the "right" one in the abstract. The one that works is the one you'll actually run for long enough to see it through a real block, with real overload and a real deload when the block ends.