In 1385, King Richard II of England invaded Scotland with his army. During this invasion, two of the king’s knights realized that they were using the same coat of arms. Richard Scrope, 1st Baron Scrope of Bolton from Bolton in Yorkshire and Sir Robert Grosvenor from Cheshire were both bearing arms blazoned Azure a Bend Or. When Scrope brought an action, Grosvenor maintained that his ancestor had come to England with William the Conqueror bearing these arms and that the family had borne them since. The case was brought before a military court and presided over by the constable of England. Several hundred witnesses were heard and these included John of Gaunt, King of Castile and Duke of Lancaster, and Geoffrey Chaucer, and a then little-known Welshman called Owain Glyndŵr. It was not until 1389 that the case was finally decided in Scrope's favor. Grosvenor was allowed to continue bearing the arms within a bordure argent for difference. Neither party was happy with the decision, so when King Richard II gave his personal verdict on 27 May 1390 he confirmed that Grosvenor could not bear the differenced arms. His opinion was that these two shields were too similar for unrelated families in the same country to bear.
From this we see that in 1389 the Earl Marshal's Court believed the addition of a bordure created sufficient difference between two otherwise identical arms born by two different families in the same country. Richard II overruled that decision in 1390, with important consequences: in my opinion, for the worse.
Families would often differentiate different branches by making simple changes in the original arms: a cadet branch might add a bordure, or change the tincture or number of the principal charge.
However, in the simple heraldry of pre-1390 England, different families could easily assume similar or even identical arms entirely independently, as Scrope and Grosvenor (and, in an earlier case involving these arms, the Cornish family of Carminow) discovered.
Richard's decision drastically limited the possible number of simple non-conflicting arms. According to the 1389 ruling, one clear difference was sufficient. You could get many variants by just changing tinctures. In pre-1389 English heraldry, six were in common use: the colors of red, blue and black the metals of gold (yellow) and argent (white) and the predominantly white fur of ermine. For reasons of contrast, a colored charge was almost always placed on a metal or ermine, and vice versa.
Under the 1389 rule, each simple ordinary had 18 variants based on tincture changes alone. Alternatives to azure a bend or could also use argent or ermine for the bend, and red or black for the field, and could reverse the scheme to double the options.
Richard's 1390 precedent greatly reduced the available options. If you needed two clear differences (or an entirely different primary charge) your choices shrunk. Each simple ordinary only allowed six variants. If the first choice was azure a bend or, a red field could only use an argent or ermine bend. If argent was chosen, sable a bend ermine was the only remaining choice. Reversing the color scheme doubled the options to a paltry six.
After 1390, English heraldry was increasingly handicapped by the limits of the 1390 Ricardian precedent, and became more and more complex to evade its rules of conflict.