The story from St Luke's gospel of the appearance of the Angel Gabriel before the Virgin Mary, to announce to her the forthcoming birth of Christ, was always a favourite subject for painters. Let's examine a few examples and see how common approaches to portraying this subject persisted over the years.
First, here is a mediaeval altarpiece, now in the Uffizi in Florence, by Simone Martini (c.1284-1344)
Next, a picture from the early Renaissance, by Fra Angelico (c.1400-1455), another Florentine,
Next, two works from the High Renaissance; the first by Sandro Botticelli (1446-1510),
and the second by Leonardo da Vinci (1452-1519)
Now a later work: an Annunciation in the Baroque style by Philippe de Champagne (1602-74)
Obviously these paintings become steadily more lifelike, and less formal and stylised, but at the same time they have a number of features in common.
The scene is usually set neither indoors nor outdoors, but in a kind of loggia or cloister. There is often garden visible somewhere. The setting appears rather opulent for a girl who is about to marry a carpenter! The visitation catches Mary by surprise: in many paintings she is seated and has just been reading a book. But what would she have been reading? It would be the prophecy in Isaiah: "Behold, a virgin shall conceive..."
Mary's colours are always the same: whatever the circumstances, she wears a red dress with a blue gown or cloak over it. Her head-covering is always white. (Sometimes in Flemish paintings her hair hangs loose, to show she is unmarried, but this is less common in Italian art). The reason for this uniformity of dress is exactly the same as why characters in modern cartoons always wear the same clothes: it is so people will recognise her!
The Archangel Gabriel usually enters from the left of the picture. He is fair-skinned, long-haired and beardless (Byzantine theologians considered angels were the equivalent of eunuchs!) and sprouts enormous bird-wings from his shoulder-blades. He wears an ankle-length gown. He is usually making a gesture of blessing towards Mary.
A few other features may also appear on the painting. Sometimes there is a single ray of light, almost like a laser, shining down from heaven onto Mary's head. There may be a white dove in flight above, symbolising the coming of the Holy Spirit. Often there is a white lily somewhere in the picture, to symbolise Mary's purity.
Here is the scene as a relief carving, from Fountains Abbey in Yorkshire
The earliest surviving pictures of the Annunciation are many centuries older than those shown here. Artistic styles may change, but the essential approach of artists to this subject has changed very little over the ages.