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When you visit an art gallery, and pause for a respectful minute in front of each painting, what do you really see? A Virgin and Child prefiguring the redemption of mankind, or a woman with no eyebrows holding an alarmingly mature baby? Is Bronzino's Allegory with Venus and Cupid (c 1540) a depiction of the pain and joy of love or a woman lounging about in front of a sheet? So often, the captions provided by galleries go no further than telling us who painted a work, when and with what. We learn nothing of the meaning. Medieval art was not designed for 21st-century eyes. The original viewers may have understood the significance of a lapdog, an ape or a pack of cards. But today, most of us need help to crack the codes. The meanings were never set in stone across Europe. But there are enough constants to help you “read” the secret signs within. Here are five works which hang in the National Gallery - and a breakdown of their probable meanings.

Allegory with Venus and Cupid (c 1540), by Bronzino

Honeycomb: A sinister figure holds a honeycomb, which alludes to a time when Cupid was stung by a bee while stealing honey. Venus dismissed her son's tears, saying his arrows wounded people far more.

A dove: Cupid tramples on a dove - for Christians a symbol of the Holy Spirit, but in antiquity another reference to Venus.

A golden apple: Paris gave Venus a golden apple as the fairest of all the goddesses. In return she gave him Helen, igniting the Trojan War.

An hourglass: Father Time's hourglass is a memento mori, reminding us that the sands of time always run out.

Rose petals: In antiquity the rose was an attribute of Venus. Here a putto is about to scatter rose petals over the goddess. Venus once gave a rose to Cupid but, worried about his mother's sexual indiscretions, Cupid gave it to Harpocrates, god of silence, to prevent gossip. The host of a Roman banquet hung a rose from the ceiling as a sign to guests that secrets were to stay in the room, as they were “under the rose” or “sub-rosa”. That is the origin of our modern ceiling roses.

A pair of masks: At Venus's feet lie two masks, a symbol of deceit and hypocrisy.

Supper at Emmaus (1601), by Caravaggio

This painting captures a scene from Luke xxiv, 30-31. Two disciples travelling to Emmaus realise their companion is the risen Jesus. “He took the bread and blessed and broke it, and gave it to them. And their eyes were opened and they recognised him.”

The fruit bowl casts the shadow of a fish, a secret sign of recognition between persecuted Christians. The disciples were “fishers of men”. And the Greek word for fish, ichthus, formed an acrostic that read, “Jesus Christ, son of God, Saviour”. You see the fish symbol today on bumper stickers. Other secret Christian symbols included the swastika, anchor and axe, all of which “hide” a cross in their design.

Back to the fruit bowl. In Christian art, apples or figs often represent original sin, as the unnamed fruit in the Garden of Eden that triggered the Fall of Man. Also in the bowl is a pomegranate, originally associated with Proserpine, daughter of Ceres, who returns every spring to regenerate the Earth. It was appropriated by Christianity to represent the Resurrection, ie, victory over original sin.

The fruit in the bowl is starting to decay - spoiling fruit or meat is a common sign that life is short and the world transient. As Ecclesiastes i, 2 puts it: “Vanity of vanities, all is vanity.” Or as Private Frazer put it: “We're doomed.” Other memento mori to look out for include blown roses, guttering candles, overturned cups and skulls.

The scallop shell worn by the older disciple is the symbol of St James, son of Zebedee, and of pilgrims generally. Travellers to his shrine at Santiago de Compostela wore the scallop badge to receive protection and alms along the way. The shell later appears in the heraldic arms of families whose ancestor made the pilgrimage, such as the Churchill, Wesley and Spencer families. Today the shell is a symbol of Methodism, and is hidden in Prince William's coat of arms as a private tribute to his mother.

The Wilton Diptych (1395-99), anonymous

This dyptych portrays the presentation of the kneeling Richard II to the Christ-child. He and the angels wear Richard's personal emblem, a white hart. He ordered it to be displayed on all public buildings, which is why England has more than 400 pubs called the White Hart.

Three saints stand beside the king, identified by their attributes. Here we have Edmund, Edward the Confessor, and John the Baptist. Edmund is holding an arrow, the weapon that will kill him. Edward holds a ring which, as legend has it, he gave to a pilgrim who turned out to be John the Evangelist in disguise.

When John the Baptist saw Jesus, he cried: “Behold the Lamb of God, who takes away the sins of the world” (John i:29). So the lamb became a symbol of the Baptist and Christ. The angels' flag, a red cross on a white background, symbolises the Resurrection. Together, the Lamb and Flag is form the name of many (more) English pubs.

The Virgin Mary is larger than everyone else, a technique for representing the most important figure. In the 13th century a costly blue dye arrived in Italy called ultramarine, made from lapis lazuli, a semiprecious stone mined in Afghanistan. This “precious” colour was quickly assigned to the mother of Jesus.

The angels wear crowns of roses, another Marian symbol. In medieval times Mary was identified as the unnamed bride in the Song of Solomon, who was “a garden enclosed, a fountain sealed up”. This was taken to mean virginity, so Mary is often referenced in Western art as a walled rose garden, or rosary.

The Arnolfini Portrait (1434), by Jan van Eyck; Marriage à la Mode II (c 1743), by William Hogarth

These two paintings contain many symbols, celebrated in the first, satirised in the latter. In the van Eyck, the wife is attended by a small dog, a sign of fidelity. In the Hogarth, a dog tugs a woman's cap from the husband's pocket, showing his infidelity. Between van Eyck's couple hangs a “spotless mirror”, a description of the Virgin Mary. In the Hogarth the wife holds up a pocket mirror, perhaps signalling to her lover. In the Arnolfinis' chandelier the burning candle may represent the light of God; in Hogarth's chandelier the candles are burnt out.

Beside the Arnolfinis' mirror hang a rosary and brush, a representation of ora et labora, prayer and work, the motto of St Benedict. Hogarth scatters cards, a symbol of vice, on the floor, and ties a lover's ribbon to the husband's broken sword - perhaps suggesting impotence, like the broken nose of the bust on the mantelpiece. Certainly the husband is unwell: the spot on his neck is a sign of syphilis. No wonder Cupid, in the picture over the fireplace, is playing the bagpipes among ruins.

Justin Scroggie is the author of Tic Tac Teddy Bears and Teardrop Tattoos (Hodder & Stoughton, £12.99)

Five symbols to look for

Small dog - fidelity

Mask - deceit, hypocrisy

Rotting fruit or meat, or an hourglass - brevity of life

Roses - Venus or the Virgin Mary

Large “beauty” spot - syphilis, can signify infidelity

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