Companies take trademarks very seriously - just look at how much they spend on them.
AT THEIR best, logos are welcome friends, like the sight of the London Transport roundel indicating a tube station on a dark and dubious street. Virtually unchanged since 1933, Edward Johnston's design classic will make its latest appearance on the six new underground stations of the pounds 3.2bn Jubilee Line extension.

On the other hand, logos can arouse intense annoyance. The high streets of Britain were scarcely enhanced when the Midland Bank replaced its familiar gryphon with the red and white lozenge of its current owner, the Hong Kong and Shanghai Banking Corporation. This spiky motif was adapted from HSBC's towering headquarters in Hong Kong. Sir Norman Foster's design, which aroused the disapproval of the island's feng shui experts, is an unlikely symbol of fiscal prudence since it is said to be the most expensive building ever constructed.

Understandably, institutions place great importance on symbols of corporate identity. Unfortunately, they often get it wrong. Few of BT's millions of customers can have warmed to BT's blue and red piper sign. This curiously fey figure, known colloquially as "man drinking a yard of ale", was a central element in the company's pounds 50m redesign in 1991. This wispy, stylised figure is curiously similar to the logos of the main UK political parties. At a distance, the stylised red rose of New Labour could be mistaken for the red and blue Tory torch, which in turn resembles the flame-like wings of the Lib Dems' yellow bird symbol. Going by their web-sites, however, we can see that the real logos of the parties are the leaders, whose features loom over the weedy symbols. The slow revelation of William Hague's Mekon- like dome is a particularly disturbing experience. The fact that institutions tend to think alike about trademarks is revealed in Marks of Excellence, Per Mollerup's visually appealing, but stodgily written, exploration of logos, which has just been re-issued in paperback (Phaidon, pounds 22.95). His taxonomy draws together those companies which, for example, use birds, flags and crowns as their symbols. Dogs are particularly popular: Greyhound buses, HMV records, the bulldog of Mack trucks, the cartoon hound on Niceday stationery and, most appropriate of all, the Scottie with wagging tail formed by the letters "Spratts." Other animal symbols range from the leaping cat on Slazenger rackets and the dromedary on Camel cigarettes, to the Qantas kangaroo and the Lacoste crocodile. (A tennis champ of the Twenties, Jean-Rene Lacoste, was nicknamed "The Crocodile".)

Logos in the form of handwriting include Coca-Cola, Ford and Harrods. Mollerup notes that the Paul Smith signature which appears on the designer's label is not what appears on Mr Smith's cheques. But he fails to point out that arguably the most famous "signature" of all, the Walt Disney logo, was not the work of the eponymous film producer. Companies whose logos make striking use of initials include Volkswagen, McDonalds, Rolls-Royce, and the burgeoning retail chain whose name commemorates the initials of its Swedish founder, Ingmar Kamprad Elmtaryd Agunnaryd.

Some trademarks have not stray- ed far from their heraldic origins, such as the EIIR monogram on Royal Mail postboxes or the BP shield. What Mollerup fails to add is that BP spent pounds 1m researching its logo in 1989. After much heart-searching, the company took the less-than-earthshaking decision to italicise the two initials. The revamp cost around pounds 100m. In contrast, the BBC recently spent pounds 5m on a corporate redesign whose main outcome was to stiffen the backbone of its previously italicised logo.

The cost of such tinkering may seem preposterous, but Mollerup, himself a design consultant, insists that "every design programme needs to be adjusted to meet changing conditions". Presumably, that's why we've lost the green gherkin and "57 varieties" from Heinz products and the clock from Crosse & Blackwell. Fortunately, Tate & Lyle had the good sense not to tamper with its tins of golden syrup, which still bear the quotation "Out of the strong came forth sweetness" (Judges 12:6) illustrated by a decaying lion corpse and swarm of bees. Though much modified, the Camp Coffee label still boasts its tartan-clan defender of the Raj. In retrospect, however, the US detergents giant Procter & Gamble may wish it had updated its "man in the moon" logo, which first appeared in 1886, before a gaggle of paranoid conspiracy-hunters denounced the trademark as a sign of corporate satanism.

Mollerup notes that the Shell scallop "would hardly be recognised as a shell if the company did not carry the name." In fact, this carefully guarded symbol looks more like a sun-rise in its latest formulation. But such simplification is not always the rule. After modernising the label, Colman's quietly re-introduced a horned bull's head on its mustard a few years ago.

The Michelin man, one of the most famous of all trademarks, has scarcely changed since 1898. The founding Michelin brothers got the idea of Bibendum from a stack of tyres at a trade show. Drawn by the poster artist O'Galop, this cheery endomorph continues to humanise the image of the tyre company in a humorous way. But I doubt if the woman whom I once saw being chased down Brixton High Street by a man in a Bibendum costume feels very warmly towards the trademark.