What are the swords called in Demon Slayer?

What is the length, weight and shape of the sword?
The total length of the sword is usually around 100 to 110 cm (approximately 39 to 43 inches) and the weight of the Nichirin Sword is usually between 1.2 to 1.5 kg (approximately 2.6 to 3.3 pounds). Most of them have a traditional katana shape, with a single-edged, curved blade. And the guard is often circular or slightly oval-shaped.
Why Nichirin Blades can kill demons?

Who make these Nichirin Blades?

Why do these swords have different colors?
“When the swordsman wields the Nichirin sword for the first time, the sword undergoes a process of recognition”, said by Tecchin Tecchikawahara, the leader of the Swordsmith Village. If the swordsman’s blade resonates, the blade changes color according to the user’s breathing style and personal attributes; Otherwise, it becomes dull.
What is the connection between different colors of sword and breathing styles?
Each Breathing Style (e.g., Water, Flame, Thunder) is intrinsically tied to a specific element or force of nature. The Nichirin Blade’s color directly mirrors this elemental affinity, and the colors are closely associated with the characters who wield them:
- Water Breathing → Blue (Giyu Tomioka : calm, fluidity, adaptability).
- Flame Breathing → Red (Kyojuro Rengoku : passion, intensity, destruction).
- Thunder Breathing → Golden/Yellow (Zenitsu Agatsuma : speed, lightning-like precision).
- Wind Breathing → Green (Sanemi Shinazugawa : sharpness, cutting force akin to storms).
- Stone Breathing → Gray (Gyomei Himejima : stability, unyielding strength).
Different breathing styles of characters and their swords
Sun Breathing
Yoriichi Tsugikuni: A legendary Demon Slayer from Sengoku era, creator of Sun Breathing, with black sword (Yoriichi Tsugikuni). His sword color is in fact black, established by association with Sun Breathing users. Not mentioned clearly in the wiki because of his significance in history, and hence such possible inferences made by associating also with Tanjiro’s black blade.
Why Nichirin Blades can kill demons?
Tanjiro Kamado: Initial training was in Water Breathing, although it mostly uses Sun Breathing, with black sword (Tanjiro Kamado). Initially, it appeared blue but turned black when pulled out for the first time-a very rare case possible points out Sun Breathing and family legacy.
Water Breathing
Sakonji Urokodaki: Tanjiro’s mentor and previous Water Hashira, equipped with a blue sword, the only standard for a Water Breathing practitioner (Sakonji Urokodaki).
Giyu Tomioka: Current Water Hashira, recognized for stoicism, has blue as well (Giyu Tomioka).
Murata: Minor Demon Slayer has a blue sword. Less covered on the series (Murata).
Sabito: Deceased. Trained Tanjiro and had a blue sword and is very significant in flashbacks (Sabito).
Makomo: Deceased. Another trainer of Tanjiro is with a blue sword, appearing in training arcs (Makomo).
Aoi Kanzaki: She is a nurse of the Butterfly Mansion and a Demon Slayer with a blue sword, though she doesn’t involve herself into fights (Aoi Kanzaki). Her role is mostly supportive, but she keeps her title of a Demon Slayer along with her sword.
Flame Breathing
Shinjuro Rengoku: The former Flame Hashira, father of Kyojuro, with an orange-red sword, now retired (Shinjuro Rengoku).
Kyojuro Rengoku: Flame Hashira whom we know from the Mugen Train arc, having an orange-red sword and fiery determination (Kyojuro Rengoku).
Thunder Breathing
Jigoro Kuwajima: Former Thunder Hashira, Zenitsu’s mentor, and this one, deceased, carried a yellow sword, so important to his legacy in training (Jigoro Kuwajima).
Zenitsu Agatsuma: Main character, with a yellow sword, fast, excels at the first form of Thunder Breath (Zenitsu Agatsuma). Kaigaku, another user, is excluded because he became a demon and thus doesn’t use a Nichiren sword anymore.
Stone Breathing
Gyomei Himejima: The Stone Hashira, blind, uses a flail and axe, not a sword, made gray, which might be surprising from a Hashira’s perspective showing such unusual design (Gyomei Himejima). This represents his unique way of fighting by leveraging his acute hearing along with strength.
Wind Breathing
Masachika Kumeno: Deceased Demon Slayer. Friend of Sanemi, associated with a green sword and is significant in Sanemi’s story (Masachika Kumeno).
Sanemi Shinazugawa: Wind Hashira, pugilistic, fierce personality, and has a green sword, standard for Wind Breathing (Sanemi Shinazugawa).
Flower Breathing
Kanae Kocho: Dead Flower Hashira, sister to Shinobu Kocho, with a light pink sword (Kanae Kocho).
Kanae Tsuyuri: Demon Slayer but currently in action, adopted sister of Kanae Kocho, has a light pink sword, amidst Flower Breathing (Kanae Tsuyuri). Note: The name ‘Kanae Tsuyuri’ is correct per the wiki though it used to confuse people with Kanae Kocho.
Insect Breathing
Shinobu Kocho: Insect Hashira, specializing in using poison, and has a lavender blue sword that is unique because of its thin needlelike design (Shinobu Kocho).
Serpent Breathing
Obanai Iguro: Known for his precision, snake-like strikes with his lavender sword for using as a Serpent Hashira (Obanai Iguro).
Love Breathing
Mitsuri Kanroji: Formerly using Flame Breathing but is now Love Breathing, has a dark pink sword, for the whip-like agility in fights (Mitsuri Kanroji). Her shifting from Flame to Love Breathing represents personal growth.
Sound Breathing
Tengen Uzui: Sound Hashira, with an amber dual-bladed sword, characterized by his explosive, flashy combat style (Tengen Uzui). The dual-blade design raises the effectiveness of his Sound Breathing techniques.
Mist Breathing
Muichiro Tokito: Mist Hashira, with a white sword, builds attacks that bewilder the enemy like mist (Muichiro Tokito).
Beast Breathing
Inosuke Hashibira: Uses two indigo gray swords as he completely embodies the dual wielding aspect in his wild, animalistic, chaotic fighting style (Inosuke Hashibira). His swords are all nicked, to saw through demons and thus empower Beast Breathing.
Anime Swords
Yoriichi Sword: Wooden Japanese Anime Katana Samurai Sword With Yellow Handle And Black Scabbard