Meaning
The kanji 凸 (totsu) strikingly visualizes the concept of a protrusion, something that bulges or sticks out, or a convex shape. Its visual form serves as a direct and intuitive pictograph. The central vertical line segment, elevated above the two shorter outer segments, clearly illustrates a raised or projecting feature. Picture a small hill or a bump on an otherwise flat surface; the kanji mirrors this exact form.
Historically, 凸 is classified as a pictographic character (象形文字, shōkei moji). Its design visually depicts a raised or projecting shape. Its direct visual representation makes it one of the most straightforward kanji to understand purely by its form. You'll often find it paired with its inverse, 凹 (ō), which means "concave" or "hollow." Together, they form the common compound 凸凹 (dekoboko), meaning unevenness or bumps and hollows.
The kanji 凸 has a stroke count of 5. Despite its simple appearance, it's classified as a Jōyō kanji (常用漢字), specifically a Grade 6 kanji taught in elementary school. However, its specialized usage, especially in technical, scientific, and nuanced descriptive contexts, means it's frequently encountered and mastered by JLPT N1 learners. Its radical is 凸 itself (radical 17), a unique feature that further emphasizes its fundamental visual meaning.
Readings
On'yomi (音読み) — Chinese-derived readings
The primary On'yomi reading for 凸 is トツ (totsu). This reading appears in many formal, technical, and scientific compound words, particularly when referring to the abstract concept of convexity or protrusion.
- 凸レンズ (totsu renzu) — convex lens
This term is fundamental in optics, describing a lens that converges light rays because of its outward curving shape.
- 凸面 (totsumen) — convex surface
In geometry and physics, this describes a surface that curves outward, much like the outer surface of a sphere.
- 凸型 (totsugata) — convex shape/mold, male die
It refers to a shape that is convex or a mold designed to produce an indentation, serving as the counterpart to a concave shape.
Kun'yomi (訓読み) — Native Japanese readings
The main Kun'yomi reading for 凸 is でこ (deko). You'll often find this reading in more colloquial expressions or specific compounds, especially when referring to a physical bulge or unevenness.
- 凸凹 (dekoboko) — unevenness, bumps and hollows, ruggedness
This is arguably the most common and important compound featuring 凸, combining it with its inverse 凹. It describes a surface that isn't smooth, possessing both raised and depressed parts.
- お凸 (odeko) — forehead
An informal and somewhat endearing term for the forehead, お凸 refers to its natural slight bulge. The 'お' (o) is an honorific prefix, adding a touch of politeness or familiarity.
- 出っ歯 (deppa) — buckteeth, protruding teeth
While 出っ歯 doesn't directly use 'deko' for 凸, the concept of 'protrusion' is strongly present. Here, '出' (de) means 'to come out' or 'protrude,' directly linking to the core meaning of 凸.
Common Words & Compounds
The kanji 凸 is crucial for forming various compound words, spanning everyday descriptions to scientific terminology. Understanding these compounds is key to grasping the full scope of 凸's meaning.
- 凸凹 (dekoboko) — unevenness, rough surface, bumps and hollows.
This fundamental compound describes a surface that isn't flat, featuring both raised (凸) and recessed (凹) parts. Example: 道は凸凹だった。(The road was bumpy.)
- 凸面 (totsumen) — convex surface.
It refers to a surface that curves outwards. Example: 車のサイドミラーは凸面鏡だ。(Car side mirrors are convex mirrors.)
- 凸レンズ (totsu renzu) — convex lens.
This type of lens is thicker in the middle and thinner at the edges, designed to focus light. Example: 望遠鏡には凸レンズが使われている。(A convex lens is used in telescopes.)
- 凸型 (totsugata) — convex shape, male die.
It describes something with a convex form or a mold that creates an indentation, serving as the counterpart to a concave shape. Example: 鍵の差し込み口は凸型をしている。(The keyhole has a convex shape.)
- 凸版 (toppan) — letterpress printing, relief printing.
This is a printing technique where the image is raised from the surface, similar to a stamp. Example: 古い本は凸版で印刷されていた。(Old books were printed by letterpress.)
- 突出 (tosshutsu) — protrusion, projection, prominence.
This means "to project," "to stick out," or "to stand out." While often written with 突, 凸出 (tosshutsu) also exists with the same meaning, emphasizing the convex nature. Example: 彼の額は少し突出している。(His forehead protrudes slightly.)
- 凸部 (totsubu) — convex part, protruding part.
This refers to a specific section of an object that protrudes or is convex. Example: 機械の凸部を確認してください。(Please check the convex parts of the machine.)
- 凸面鏡 (totsumenkyō) — convex mirror.
This is a mirror whose reflective surface curves outwards, often used for wider fields of view. Example: コンビニの防犯カメラは凸面鏡を使っている。(Convenience store security cameras use convex mirrors.)
- お凸 (odeko) — forehead.
This is a common, informal term for the forehead, especially one that is prominent. Example: 彼は広いお凸が特徴だ。(He is characterized by a broad forehead.)
- 凸版印刷 (toppan insatsu) — letterpress printing.
This is a more complete term for the printing method where raised characters transfer ink. Example: 昔ながらの凸版印刷は独特の風合いがある。(Traditional letterpress printing has a unique texture.)
- 凸状 (totsujō) — convex shape/form.
Describes something that has a convex form or shape. Example: この石には自然な凸状の部分がある。(This stone has a naturally convex part.)
Example Sentences
この道は凸凹が多いので、運転には注意が必要です。
Kono michi wa dekoboko ga ooi node, unten ni wa chūi ga hitsuyō desu.
This road has many bumps and hollows, so caution is needed when driving.
凸レンズを使うと、光が一点に集まります。
Totsu renzu o tsukau to, hikari ga itten ni atsumarimasu.
When you use a convex lens, light gathers at one point.
車のサイドミラーは視野を広げるために凸面鏡が使われている。
Kuruma no saido mirā wa shiya o hirogeru tame ni totsumenkyō ga tsukawarete iru.
Car side mirrors use convex mirrors to broaden the field of view.
彼のお凸は広くて、賢そうに見える。
Kare no odeko wa hirokute, kashikoso ni mieru.
His forehead is broad, and he looks intelligent.
この机の表面は少し凸凹しているので、書きにくい。
Kono tsukue no hyōmen wa sukoshi dekoboko shite iru node, kakinikui.
The surface of this desk is a bit uneven, so it's hard to write on.
この石には自然な凸状の部分がある。
Kono ishi ni wa shizen na totsujō no bubun ga aru.
This stone has a naturally convex part.
彼は生まれつき出っ歯で、少し歯並びが突出している。
Kare wa umaretsuki deppa de, sukoshi hanarabi ga tosshutsu shite iru.
He has buckteeth by birth, and his teeth protrude slightly.
指紋は皮膚の表面の細かい凸凹によって作られている。
Shimon wa hifu no hyōmen no komakai dekoboko ni yotte tsukurarete iru.
Fingerprints are formed by the fine ridges and valleys (unevenness) on the skin's surface.
古代の彫刻には、顔の凸部が誇張されているものも多い。
Kodai no chōkoku ni wa, kao no totsubu ga kochō sarete iru mono mo ooi.
Many ancient sculptures exaggerate the convex parts of the face.
Memory Tip
The kanji 凸 (totsu) is incredibly visually intuitive. To remember it, simply look at its shape: it literally depicts a bump or a protrusion. Picture a small hill, a speed bump on a road, or a raised button. The central vertical stroke stands out above the two shorter horizontal strokes on either side, clearly illustrating something sticking out or convex. You can also think of it as a person's forehead (お凸), which naturally has a slight bulge. Its inverse, 凹 (ō), meaning concave, looks exactly like a hollow or an indentation. This makes the pair very easy to distinguish and remember once you grasp their visual connection to their meanings.