Delving into AD&D: Everything you want to know about First Edition Combat.

Gary Gygax wrote some great prose in first edition ADnD. But there were sections where things were not so clear. One of them was how initiative is supposed to work in combat. Luckily a couple of years back a guy named David Prata wrote A.D.D.C.I.T. which lays out the entire first edition combat in detail backed by footnotes.

It may not be the definitive interpretation of ADnD combat but is certainly one of the most complete. For me personally after reading it I finally got what Gygax was getting at in the 1st edition DMG.  The two key points are

• The initiative number is the segment when the OPPOSING side gets to act. So if Side A rolls a 5 and Side B rolls a 2; Side A starts their combat round on segment 2 and Side B starts their combat round on segment 5. Thus for side A any spell that takes 2 segments or less can be cast safely. Three segments spells will be simultaneous with melee combat. Any longer casting time spells run the risk of being disrupted by Side B.
• Weapon speed, and all the rules about multiple attacks with weapon speed only come into play when initiative is tied.  So when Side A and B both roll a 4 then weapon speeds are compared and it may be possible that if the weapon speed is low enough the wielder of that weapon will get multiple attacks.