The most recent meeting of the Oxford University Heraldry Society was addressed by Steve Slater from Wiltshire on the subject of the heraldry associated with bastards.
Althogh the bend sinister is the classic symbol of bastardy in heraldry there are really no fixed conventions about the matter, which for some might be sensitive, but the higher up the social scale the more people were prepared to flaunt heraldically their parentage - to be bastard may be awkward, but being aroyal bastard has a cachet all its own - and medieval kings often found their illegitimate offspring more relaible than their legitimate ones
Earl William's effigy still has traces of colour to indicate that the arms were of the same tincture. This is an early instance of the heritability of arms, even if it bypasses a generation.
Effigy of William Longespee, Earl of Salisbury
Royal bastards offer some of the best examples. Sir Roger de Clarendon, illegitimate son of Edward Prince of Wales, derived his arms from the arms of peace borne by his father, and bore Or, a bend noir with three ostrich plumes.
Following their partents marriage in 1396 they were legitimised by both the Pope and the King, and used a coat of arms of France modern and England quarterly ( the Royal arms as used by their half brother King Henry IV) with a bordure company argent and azure:
Following the marriage of Joan Beaufort to King James I of Scots in 1425 the bordure company became, somewhat ironically given that for the Beauforts it was a sign of legitimacy, the norm in Scotland for denoting bastardy.
These early examples tend to show a very narrow bend sinister, minimising its visual effect, and there appears to have been a practice whereby after three generations the bend sinister was discarded. Although not part of official heraldic practice this appears to have been tacitly accepted. Similarly Charles Somerset, first Earl of Worcester, bastard son of Henry Beaufort Duke of Somerset, originally bore his father's arms with a bend sinister, or, on a gold ground the Beaufort arms on a fess, but subsequently he, and certainly his descendants, the Marquesses of Worcester and Dukes of Beaufort, have borne the undifferenced arms of the medieval Beauforts.
The bend sinister came to be shortened to be a baton in the centre of the shield.
The eldest of King Charles II's illegitimate sons, the Duke of Monmouth bore two successive coats of arms, both of which were differenced by a baton sinister. In his case it was plain white, comparable to the label of a legitimate eldest son.
The heraldic blazon is: Quarterly: 1st and 4th grand quarters, the Royal Arms of King Charles II (viz. quarterly: 1st and 4th, France and England quarterly; 2nd, Scotland; 3rd, Ireland); the whole within a bordure company argent charged with roses gules barbed and seeded proper and the last; overall an escutcheon gules charged with three buckles or (the Dukedom of Aubigny); 2nd grand quarter, argent a saltire engrailed gules between four roses of the second barbed and seeded proper (Lennox); 3rd grand quarter, quarterly, 1st, azure three boars' heads couped or (Gordon); 2nd, or three lions' heads erased gules (Badenoch); 3rd, or three crescents within a double tressure flory counter-flory gules (Seton); 4th, azure three cinquefoils argent (Fraser).The Earls of Munster (title created 1831 and extinct in 2000), descended from King William IV and Mrs Jordan, bore the King's arms, with the inescutcheon of Hanover, differenced with a baton azure, charges with three anchors or.
English arms difference the supporters and the crest was well as the arms, but this is not the practice in Scotland.