Overview
The Codex Astartes is a vast tome detailling the tactics, orgainization, markings and rituals of the Space Marines. Written after the Horus Heresy by the Primarch of the Ultramarines Robulte Guilliman the Codex is followed without question by the Ultramarines and ther sucessors. Many other chapters also follow the Codex, though not always as striclty as the Ultramarines, and all chapters that do follow the Codex are known as Codex Chapters.
Organizational Doctrine
The Codex states that a Space Marine Chapter should be spilt into 10 companies of 100 Marines each, plus a Space Marine Captain, Apocthecary, Company Standard and Chaplain for each Company .
The 1st Company or Veteran Company consists of Veteran Squads and/or Terminator Squads. Support for the 1st company consists of Land Raiders and Dreadnoughts. Only the 1st Company may use the hallowed suits of Terminator Armour
The 2nd, 3rd, 4th and 5th Companies are known as Battle Companies and all have the same organization. They are split into 6 Tactical Squads, 2 Assault Squads and 2 Devastator Squads. The Assault Squads can instead be deployed on Space Marine Bikes or Land Speeders. Battle Companies often use Rhino and Razorback transports, and commonly have Dreadnoughts.
The '"6th, 7th, 8th and 9th Companies"' are known as Reserve Companies. The 6th and 7th are comprised entirely of Tactical Squads, the 8th of Assault Squads and the 9th of Devastator Squads. Like the Battle Companies the Reserve Companies also make extensive use of Rhino and Razorback transports, though Dreadnoughts are usually only found in the 9th Company. The 6th Company can be entirely deployed on Space Marine Bikes and the 7th on Land Speeders. The 8th Company can use either Space Marine Bikes or Land Speeders.
The 10th Company or Scout Company consits entirely of Scouts and is often not 100 strong as recruitment does not provide a steady flow of new recruits. As such the 10th Company is the most lightly armed and is most often used as a recon force. Scouts are sometimes mounted on Space Marine Bikes but do not use Rhino or Razorback transports.
In addition to the 10 Companies each chapter has an Armoury consisting of the chapter's Techmarines and tanks, a Librarium cosisting of the Chapter's Librarians, a Chapter Fleet and the Chapter Master, plus various headquaters staff and the Chapter's Servitors and human serfs.
Heraldry
Each Chapter has its own unique colour scheme and Chapter Badge however Codex Chapters all follow a common heraldry system, though there are many slight variations from the system below.
Company Colours - Each Company has a unique colour that its members wear, commonly on the shoulder pad rims, but some chapters use chest eagles, bolter cases, knee pads, helmets or other parts of a Space Marine's Power Armour.
1st Company - White or Silver - Memebers of the 1st Company should also paint their helmet the company colour.
2nd Company - Yellow or Gold
3rd Company - Red
4th Company - Green
5th Company - Black
6th Company - Orange
7th Company - Purple
8th Company - Grey
9th Company - Blue
10th Company - Nominally white, however the company colour is not displayed in the members armour because they are Scouts.
Squad Markings - The left shoulder pad of a Space Marine in power armour always shows the Marine's Chapter Badge. The right pad shows the squad badge, this shows the type of squad the marine belongs to and the number of his squad. Veteran Squads use a Maltese Cross, Tactical Squads use an arrow, Assault Squads use four perpendicular arrows pointing outwards and Devastator Squads use an inverted V. Space Marines in terminator armour wear the Crux Terminatus on their left pad, and their Chapter Badge on their right pad.
Officers and Specialists
Space Marine Captains - Wear the heraldry of their company, sometimes embelished with their own personal heraldry
Librarians - Wear blue armour, irrsepecive of what chapter they belong to, and wear their Chapter Badge as normal.
Chaplains - Wear black armour, irrsepecive of what chapter they belong to, and wear their Chapter Badge as normal.
Techmarines - Wear red armour, irrsepecive of what chapter they belong to, and wear their Chapter Badge as normal.
Apocthecaries - Wear white armour, irrsepecive of what chapter they belong to, and wear their Chapter Badge as normal.
Sergeants - Wear the same hearaldy as their squad, but carry the squad's banner, which displays the Chapter Badge and the squad's number, plus a red skull (the symbol of a sergeant). Sergeants often paint their helmets red, or in some cases wear a red skull on their helmet instead.
Veteran Sergeants - Wear the same hearaldy as their squad, but carry the squad's banner, which displays the Chapter Badge and the squad's number, plus a red skull (the symbol of a sergeant). Veteran Sergeants often paint their helmets red and add an additional white stripe or laurel wreath.