Regular, Mechanical Repetition
ごとに marks a fixed interval or an exhaustive "each one".
- 三日ごとに水をやってください。(Water it every three days.)
Unlike たびに, it carries no feeling. It is a schedule or a distribution.
| Pattern | Sense |
|---|---|
| たびに | every occasion, often emotional |
| ごとに | a regular interval or each member of a set |
会うたびに嬉しい works. ×会うごとに嬉しい is odd — meeting is not a scheduled interval.
The "Each One" Sense
With a countable noun it distributes across every member:
- 国ごとに習慣が違う。(Customs differ from country to country.)
This overlaps with によって in its "depending on" sense. ごとに stresses that you can go through them one by one; によって stresses that the outcome varies.
Against おきに
The pair that catches everyone:
| Pattern | Meaning |
|---|---|
| 三日ごとに | every three days |
| 三日おきに | every fourth day — skipping three |
おきに counts the gap; ごとに counts the cycle.