Counter Sound Changes: The Master Pattern

Most learners meet counters one at a time and memorize each one's "irregular" readings from scratch — ippon, sanbon, roppon for 本; ippiki, sanbiki, roppiki for 匹; and so on, as if each counter had its own private chaos. It doesn't. Nearly every counter irregularity in Japanese is the output of just two euphonic rules applied to the counter's first consonant. Learn the two rules here, look at what a counter starts with, and you can predict its whole paradigm — even for counters you've never seen. This is the reference page the rest of the counter guide points back to; it is worth reading slowly once and returning to often. The same two forces also reshape the numbers themselves (三百 sanbyaku, 六百 roppyaku), covered on Sound Changes in Numbers.

The two rules

Rule 1 — Gemination (促音化), after 一・六・八・十. These four numbers historically ended in a stop consonant, so the boundary tightens into a small っ and the counter's first consonant doubles. If that consonant is h, it hardens further to p. This is why 一本 is ippon and 六階 is rokkai.

Rule 2 — Voicing (連濁), after 三・何. These two end in the moraic ん, and a following h softens to b (三本 sanbon, 何杯 nanbai). The sole maverick is 分, which takes p instead of b.

Everything below is these two rules meeting counters that begin with different consonants. The numbers 2, 4, 5, 7, 9 are phonologically "quiet" — they trigger neither rule — so 二本 ni-hon, 五杯 go-hai, 七冊 nana-satsu are all perfectly regular (with one exception for 分, noted below).

本 (hon) — the model h-counter, fully declined

本 begins with h, so it shows both rules at full strength: gemination hardens h to pp, and voicing softens it to b.

匹 (hiki) — the same paradigm, fully declined

匹 also begins with h, so it moves in lockstep with 本. Learn one and you've learned the shape of the other.

The master grid

Here is the payoff — six common counters, all triggered by the same two rules. The rows that change are always the same numbers; only the result differs by initial consonant.

Scan the columns and the logic jumps out. The four h-counters (本, 匹, 杯, 分) share an identical shape: p on rows 1/6/8/10, a soft consonant on rows 3/何 — b for three of them, and p for the lone maverick 分. The k-counter 階 geminates the same way but voices k → g after ん. The s-counter 冊 geminates on 1/8/10 but not on 6, and it doesn't voice after ん at all.

By initial consonant

h-counters (本, 匹, 杯, 分, 百) — the busiest

These change the most, because h is unstable in both environments. Gemination hardens it to pp; voicing softens it to b. The whole family — 本 hon, 匹 hiki, 杯 hai, 分 fun, and the place-word 百 hyaku — moves together.

The one maverick: 分 takes p, not b, after ん. Where 本 gives sanbon and 杯 gives sanbai, 分 breaks ranks: 三分 is さんぷん sanpun, 何分 is なんぷん nanpun, and — because 四 (yon) also ends in ん — 四分 is よんぷん yonpun. There is no meaning-based reason; it is simply a fact about 分. See Telling Time for 分 in full.

k-counters (階, 回, 個, 軒)

Gemination is clean: 一階 ikkai, 六階 rokkai, 八回 hakkai, 十個 jukko. Voicing after ん, though, is a near-dead force among k-counters: only 階 still shows it (三階 sangai, 何階 nangai), and even that is receding — many younger speakers say さんかい sankai. Its cousins do not voice: 三回 is sankai, 三個 is sanko. So treat 三階 = sangai as the traditional reading, sankai as a widely accepted modern variant, and every other k-counter as unvoiced after ん.

See Floors and Occurrences for 階 and 回 in detail.

s-counters (冊, 歳, 足) and t-counters (頭, 点, 通)

These are the "quiet after ん" group. They geminate on 1/8/10 but skip 6: 一冊 issatsu, 八冊 hassatsu, 十冊 jussatsu, but 六冊 stays a plain ろくさつ roku-satsu (exactly like 六千 roku-sen). And they do not voice after ん: 三冊 is san-satsu, 三頭 is san-tō. The one frozen survivor of s-voicing is the place-word 千 → sanzen; ordinary s-counters like 冊 never follow it.

Two variant readings to recognize

• 十 before geminating counters: jup- or jip-. Modern standard Japanese says じゅっ — juppon, jukkai, jussatsu. Dictionaries and older prescriptive sources list じっ — jippon, jikkai, jissatsu. Both are correct; juppon is what you'll hear.
• 三階 / 何階: sangai or sankai. As above — voicing is traditional, sankai is the spreading modern form. Neither will be misunderstood.

Common mistakes

Key takeaways

• Two rules generate nearly all counter irregularity: gemination after 一/六/八/十 and voicing after 三/何.
• The counter's first consonant decides the result: h → pp (geminated) or b (voiced); k → geminated, sometimes g; s/t → geminated but quiet after ん.
• Maverick to memorize: 分 takes p after ん (三分 sanpun, not sanbun).
• Before s and t, 六 does not geminate (六冊 roku-satsu, 六頭 roku-tō); before h and k it does (六本 roppon, 六階 rokkai).
• Variants to accept: juppon / jippon, and sangai / sankai.