Historical context

Money: pounds, shillings and pence

English money was counted in pounds, shillings and pence. Wills, inventories and accounts write sums in three columns, often in roman numerals.

The system

12d = 1s: Twelve pence make a shillingP&Y p. x
20s = £1: Twenty shillings make a pound, so a pound is 240 penceP&Y p. x
li. s. d.: Pounds, shillings and pence, from the Latin librae, solidi, denarii. The pound is written li., l. or £. See moreD&K-S p. 21; P&Y p. x
ob.: A halfpenny, from the Latin obolus See moreWright
q. or qa.: A farthing, a quarter of a penny, from the Latin quadrans See moreWright; P&Y p. x

Two sums turn up so often that they have names of their own

A mark: 13s 4d, two-thirds of a pound. A sum of money, not a coin. See moreWright
A noble: 6s 8d, half a mark. Once the value of a gold coin, and still used as a sum.Challis

Three lines of an inventory and their total. Add each column from the right, carrying as you go.

An inventory total, worked
	li.	s.	d.	In figures
Item a cowe	iij	vj	viij	£3 6s 8d
Item a table		xiij	iiij	13s 4d
Item ij pottes		v	vj	5s 6d
Summa	iiij	v	vj	£4 5s 6d

P&Y p. x

Pence: 8 + 4 + 6 = 18d, which is 1s 6d. Write 6 and carry 1 shilling.P&Y p. x
Shillings: 6 + 13 + 5, plus the 1 carried, = 25s, which is £1 5s. Write 5 and carry 1 pound.P&Y p. x
Pounds: 3 + the 1 carried = £4P&Y p. x
Summa: Latin for the total. It heads the line that adds up a page or an account.Wright
Counters, not pens: Clerks often added on a board with counters, not on paper. Groups of dots in the margins of accounts can be a record of it.Hector pp. 40–41

D&K-S: G. E. Dawson and L. Kennedy-Skipton, Elizabethan Handwriting 1500–1650 (1966)
P&Y: J. F. Preston and L. Yeandle, English Handwriting 1400–1650 (1999)
Hector: L. C. Hector, The Handwriting of English Documents (1958)
Wright: A. Wright, Court-Hand Restored (10th edn, 1879)
Challis: C. E. Challis, ed., A New History of the Royal Mint (1992)

Facts from these sources, in our own words. Page numbers are the printed pages.