There are three ways of creating a peerage
(Technically speaking there is also a fourth, in that a peerage may also by created by an
Act of Parliament as with the
Duchy of Cornwall, but such examples are understandably rare.)
A Writ of Summons is simply a direct summons by the sovereign to attend Parliament; a letter requiring the addressee to turn up at the specified location on the given date. Letters Patent are simply an official document setting out the terms of the grant of a peerage. Creation by Tenure came about through the ownership of a particular designated territory; a method now generally considered to be non-existent except for possibly the special case of Arundel.
The creation of a peerage
The existence of peerages held by tenure has now been denied by the law on the grounds that it is incompatible with the idea of a peerage as a personal dignity. As decided in the Fitzwalter case of 1669;
and the nature of a barony by tenure being discussed, it was found to have been discontinued for many ages, and not in being, and so not fit to be revived or to admit any pretence or right of succession thereupon.
Notwithstanding the above legal opinion, historically speaking all peerages were first created by tenure, in that it was those feudal tenants-in-chief of the Crown whose attendance was required at the Curia Regis that came to be regarded as holding peerages; particularly as the idea of a Parliament and specifically a House of Lords to which peers had a specific hereditary entitlement to attend had not yet developed. When in the 13th and 14th centuries it became common for sovereigns to call Parliaments and to summon the leading magnates to attend Parliament by a writ of summons, such writs were issued as merely an incident of tenure.
It was during the reign of Richard II (in the year 1388 to be precise)that the first examples were seen of a grant of peerage by way of Letters Patent, which was a grant entirely based on the grace and favour of the Crown and not founded on any right by tenure of land. By the use of Letters Patent the Crown, (whether it intended to or not, established the idea that a peerage was purely a personal dignity, conferred by the sovereign on a deserving subject .
Creations by writ of summmons and creations by Letters Patent continued to be used side by side until the middle of the sixteenth century when the former method fell into disuse, and since the Restoration there have been no new creations by writ of summons alone, except in the specific case of a writ in acceleration; which is a method of calling the heir of a peerage to the House of Lords in the name of one of his father's subsidiary titles. (And therefore simply accelerating an individual's promotion to a seat in the House of Lords.)
From the Restoration onwards all new creation were by Letters Patent and were distinguished from the older creations by writ by virtue of the fact that different rules were applied in terms of how such titles could be inherited as explained below.
The inheritance of a peerage
The general rule regarding the inheritance of a peerage is that there is no general rule; it depends entirely on what terms were specified when the title was first granted.
As far as creations by Letters Patent are concerned it all depends on what exact terms are specified in that document. It was usually the case that Letters Patent would limit the inheritance of the title to the "heirs male of the body" of the original grantee. Therefore the title could only pass to the sons of the original title holder, or their sons and so on and so forth.
When the original grantee of the peerage title died the eldest son succeeded, or if he had previously died then his sons, the eldest having priority over the youngest. If there were no surviving male issue of the eldest son, the title would pass to the next eldest son and his issue and so forth. If it came to pass that their were no male descendants of the original grantee left living, then the title became extinct and reverted to the crown.
Although the limitation to heirs male was normal, it was not uncommon for peerages to be granted with other limitations. There are examples of peerages being granted by Letters Patent with sons-in-law, nephews or brothers being specified as the next in line, all depending on the particular personal circumstances of the original grantee.
As regards peerages created by writ of summons; a writ was simply a letter requiring the addressee to attend Parliament and therefore never contained any details regarding how the title might be devolved upon future generations. Indeed it is debatable whether such summonses where ever originally conceived with the intention of creating a specifically hereditary dignity in the first place.
However the idea was accepted that the issue of writs had created such a right and it therefore also became generally became accepted that this right was inherited in accordance with the rules of Common Law, and could therefore pass to 'heirs general'. Which is to say, that in the absence of any male heirs the title could pass to a woman.
Therefore there are a number of examples of daughters becoming Countess of Somewhere or Baroness Something 'in their own right'. They could not, and did not, of course actually attend Parliament or anything of that nature; normally their husbands would claim to be Earl of Somewhere by 'right of their wife' and their eldest son (if there was one) would inherit the title in due course.
It is a matter of opinion as to whether, in such circumstances, the husband became the first of a new line of holders of a particular title, or the nth holder of the original line. On the whole, the former appears to be the more common treatment.
A brief explanation of abeyance
It is worth noting that under Common Law the basic rule of primogeniture, that the eldest son or his male heirs inherited everything, does not apply in the case of inheritance by females. If the holder of a title died leaving only daughters for example, then each of the daughters inherited an equal share in their father's estate, including any titles he held. Since each daughter had an equal right to the title, but by definition the personal dignity of a peerage could not be shared, none of the daughters actually inherited the title. It was said to go into abeyance, placed in suspense until such time as death or inter-marriage produced one single unequivocal male heir.
It appears to have been accepted that the sovereign could terminate an abeyance in favour of one of the heirs, which appears to have been almost universally the practice to begin with. For example, when Marshal line of the Earl of Pembroke died out, the remaining heirs were the five daughters of the original William Marshal; by rights that title together with the others held by the last Marshal Anselm should have fallen into abeyance. Whilst the title of Earl of Pembroke went to Warin de Munchesni who married one of the daughters, the hereditary title of Marshal of England went in another direction to the Bigod family who had married another of the heiresses.
Strict application of the rules of abeyance appears to be a more modern phenomenon; from the seventeenth century onwards a more legalistic approach was taken to the matter of inheritance with less discretion permitted to the sovereign in the matter. Abeyance became more common, there have been occasions when a title has gone into abeyance for a century or more.
Note that none of the above applies to titles in the Peerage of Scotland. Scottish peerages are all inheritable by heirs general, which is to say that with the failure of male heirs, females may succeed, and in contrast to the above, the eldest female has priority over the youngest, so that Scottish titles do not fall into abeyance.
SOURCES
The 1911 Encyclopedia Brittanica entry for PEERAGE
See http://1911encyclopedia.org/index.htm
Charles Arnold Baker The Companion to British History (Longcross Press, 1996)