The Welsh numerical system uses a hybrid system of counting. The traditional system using units of 5, 10, and 20 (un ferch ar bymtheg) can still be found in very formal Welsh, but is predominantly used to tell the time and for dates on a calendar. A more modern system which reflects the gender of the noun has been introduced, but while correct, it does not sit comfortably with Welsh speakers (un deg dwy ferch). The most common system now in use is based on the plural form of the noun, which is non-feminine, and as such has no gender (un deg dau o bunnau).
This less than straightforward explanation is probably the strongest argument for why such a book as this is needed as it sets out step by step how each of these systems is compiled and how they are used to tell the time, name dates and years, and how to set down sums of money, and other numerals, in words.