Guess the font

One letter throws away most of the wrong answers. A double-storey g — two bowls joined by a neck — belongs to most serif text faces and to the humanist sans, Gill Sans among them. A single-storey g, one bowl with a tail hanging off it, belongs to Helvetica, Futura, Century Gothic and the geometric family generally.

After the g, the lowercase a. A single-storey a — a circle with a stem — is almost always geometric or a face aiming at something handwritten. Everything else keeps the little hood on top.

Then the capital R and its leg: straight, curved, kicked out at the end. Designers put personality in the R because it is one of the few capitals with a spare limb.

They are close enough that most people give up, and they are separated in about three seconds if you know where to look.

Capital R. Helvetica's leg comes down straight then flicks. Arial's leg is a diagonal that just runs away from the bowl.

Capital G. Helvetica has a spur — a small vertical downstroke where the crossbar meets the curve. Arial has none.

Lowercase t. Arial cuts the top of the stem at an angle. Helvetica cuts it flat.

Capital Q. Helvetica's tail is a straight stroke crossing the circle. Arial's is a wave.

The same method works for most near-identical pairs: find the letters where the two designers had to make a decision, and ignore the rest.

Grotesque and neo-grotesque sans — Akzidenz, Helvetica, Univers. Tight apertures, horizontal terminals, no obvious enthusiasm. The default look of a train station.

Geometric sans — Futura, Avenir, Century Gothic. Built out of circles and straight lines, single-storey a and g, a very round o.

Humanist sans — Gill Sans, Frutiger, Myriad. Sans-serif shapes with the proportions of a written letter; open apertures, some variation in stroke weight.

Old-style serif — Garamond, Caslon. Angled stress, gentle contrast, a small x-height.

Transitional and Didone — Baskerville, then Bodoni and Didot. The contrast between thick and thin gets sharper until the thin strokes are hairlines and the serifs are flat.

Slab serif — Clarendon, Rockwell. The serifs are the same weight as the strokes. Impossible to miss, easy to name.

Point size measures the space a letter is allowed to occupy, not the letter. What you actually read is the x-height: the height of a lowercase x against the capitals.

Helvetica has a large x-height, so it looks big, sturdy and a little crowded. Futura has a small one with long ascenders, so at the same setting it looks delicate and airy and slightly too small.

This is the first thing to judge in a photograph, before any single letter. A tall x-height with short ascenders points at signage and newsprint faces. A small x-height with long ascenders points at book faces and the geometric sans.

The end of a stroke is where a face gives itself away. A ball terminal — a little blob on the a, c, f or y — belongs to Caslon, Baskerville and most of the warmer serifs. A sheared terminal, cut at an angle, suggests speed and belongs to the humanist side.

A terminal cut dead horizontal is the mark of the neo-grotesques, and it is the single quickest way to separate Helvetica from almost anything trying to imitate it.

Look too at the aperture: how far the c, s and e stay open. A nearly closed c is Helvetica. A wide open one is Frutiger or something designed to be read at a distance.

Find the odd characters. The ampersand, the question mark, the dollar sign and the numerals carry more personality than any letter, and a designer who copied someone else's alphabet rarely bothered to copy the punctuation.

Check whether the numerals sit on the line at a single height, or hop above and below it. The hopping kind — old-style figures — nearly always means a book face rather than a screen face.

And consider the date. A sign made in 1975 is not set in a typeface released in 2004, however similar it looks; knowing roughly when something was printed removes most of the candidates before you have examined a single letter.