Flag of Saint David
The flag of Saint David (Welsh: Baner Dewi Sant) is normally a yellow cross on a black field, although it has also appeared as a black cross on a yellow field, or with an engrailed cross. It has been used as a flag representing Wales (as an alternative to the Red Dragon flag), in the same sense that the crosses of Saint George, Saint Andrew, Saint Patrick and Saint Piran are used to represent England, Scotland, Ireland and Cornwall (of which they are, respectively, patron saints). It is similar to the arms of the Diocese of St David's.
Use of the flag 
The history of the flag is somewhat ambiguous, though it seems to have emerged at the beginning of the 20th century. One theory is that it was developed to fly atop Anglican churches in Wales, in the same way that the St George's Cross was flown outside churches in England, though since 1954 churches are more likely to fly a flag bearing the armorial bearings of the Church in Wales, granted that year.
In any case, the colours of the flag - black and gold - have certainly long been associated with the Welsh saint, even if not always in the form of a symmetrical cross. St David's University College, Lampeter (now the Lampeter campus of the University of Wales, Trinity Saint David), founded in 1822, adopted these colours as the 'college colours' in 1888, and the flag of St David continues to be associated with the college, and is often flown today in a form defaced to include the cinquefoils of the crest of St David's College.