William goes on the cover in full — by the goodnight kiss he's just Will

A determined protector: where William comes from

William is what happened when the Normans crossed the Channel. The name began as the old Germanic Willahelm — wil, meaning will or desire, joined to helm, a helmet — so it adds up to something like "determined protector": a boy who wants things and shields people. It landed in England in 1066 with William the Conqueror, and the country took to it with astonishing speed. Within a couple of generations it was the most common boys' name in the land, so widespread that English is still covered in its fingerprints — every Williams, Wilson, Wilkinson and Willis in the phone book descends from some medieval Will.

The name never really left the front row after that. England has crowned four King Williams, and the current heir to the British throne carries the name today. The most quoted writer in the English language signed his plays William Shakespeare — and was fond enough of his own first name to pun on it in the Sonnets: "thou hast thy Will." It is even one of the rare classic names with a flower to its credit: sweet william, the cottage-garden bloom, has carried it since the sixteenth century.

A thousand years after it arrived, William still sits in the top handful of boys' names across the English-speaking world. Some names ride trends; this one simply outlasts them — which is why it looks so much at home in a printed title.

The name travels well, too. French made it Guillaume, German kept Wilhelm, Spanish says Guillermo and Italian sings Guglielmo — a whole map of cousins that all began as the same two Germanic syllables, while English held on to the W and the double l. Wherever your family has roots, there is probably a version of this name somewhere in the tree, which makes the English William feel less like a fashion and more like an inheritance.

Will, Bill, Billy — the many sizes of William

• William — The full dress uniform. It comes out on the first day of school, on official forms, and in that slow, three-syllable way parents say it when crayon art has appeared on the wall. It is also the natural choice for a book cover — formal enough to look printed, warm enough to still be his.
• Will — The everyday size: quick enough for the playground, grown-up enough that he will never outgrow it. If the full name feels too formal for a bedtime story, this is the form to print in the book — the story is then written around Will from the first line.
• Bill — The historic rhyming nickname. Medieval England had a fashion for rhyming pet forms, and Will became Bill the same way Robert became Bob. Rarer for small boys today, which makes it feel like a grandfather's privilege.
• Billy — The small-boy version with extra bounce — the one shouted across backyards and birthday parties. Many Williams are Billy until roughly the day they decide they aren't.
• Wills — The affectionate family form with an s on the end, used at home more than anywhere else — the shape the name takes when it's being called upstairs for dinner.