The word 'tithe' not only means 'tenth' (specifically of money or goods) - it once *was* the English word for 'tenth'.

It comes from Old English teoþa, which was just an everyday adjective.

Modern 'tenth' is a later innovation, created to better resemble the number 'ten'.

But if teoþa was the ordinal adjective for the number 'ten' in Old English, what happened to the N?

We see the same thing in other numbers:

seofon (7) > seofoþa (7th)
niġon (9) > niġoþa (9th)

Why does the N disappear when -þa is added?

Welcome to the world of the Ingvaeonic Nasal-Spirant Law! (A very long and unsexy name for a nonetheless very sexy sound change)

This change occurred within the Ingvaeonic languages, a sub-group of the Germanic family, which includes English and Frisian - but not German.

I mention German, because, if we bear in mind that German did not undergo this sound change, lots of connections between English and German start to appear, or at least make more sense!

So, how does the change work?

As its name suggests, it involves nasal sounds (such as [n] or [m]) and spirants, otherwise known as fricatives. These include common English consonants like [f], [s] or [θ] (written 'th').

Simply put, when a nasal came before a spirant in a word, the nasal sound disappeared. This left only the spirant and a lengthened vowel before it.

This is what happened in teoþa!

When -þa (which has a spirant sound) was added to the number ten, the nasal [n] had to go.

This loss of nasals before spirants can explain the differences between so many related English and German words!

For example, English has the words mouth, goose, soft, us, and five - for which the German is Mund, Gans, sanft, uns and fünf.

... English wish vs. German Wunsch, tooth vs. Zahn, other vs. ander, and so on, and so on...

This thread was inspired by my visit to the magnificent medieval tithe barn in Bradford-upon-Avon in September. It is b i g and very lovely.

I shared my 'tithe' facts with my dad at the time, but I don't think he was listening, so I resolved to inflict them on Twitter instead.