This section is from the book "Real Property, An Introductory Explanation Of The Law Relating To Land", by Alfred F Topham. Also available from Amazon: The New Law Of Property.
On the marriage of A, the land is granted to A for life with remainder to his eldest son in tail, with remainder to A's second son in tail and so on.
When A has a son, B, B becomes tenant in tail, and A becomes the protector of the settlement. Therefore as long as A is alive, B cannot bar the whole entail without A's consent. And of course B cannot bar it while he is under 21. So that - if B dies under 21, or before he has barred the entail, the land will go to B's children, or if he has none, then to the second son of A in tail, and so on.
2 Ch. G42. If three protectors are appointed and one dies, the survivor can act alone. Be Bayley-Worthington,  1 Ch. 26. The consent of the protector may be given after the death of the tenant in tail. Whitmore Searle v. Whitmore Searle,  2 Ch. 332.
Usually, however, A has some influence over his sons on their attaining 21, and he persuades his eldest son, on attaining 21, to join with him in a new deed of settlement as follows: -
When A's eldest son, B, reaches 21, A and the son execute a deed enrolled and bar the entail, and then re-settle the land on A for life, remainder to A's son B for life, remainder to B's eldest son in tail, and so on.
This is done every time that an eldest son reaches 21, and the land may thereby be tied up in a family for generations.
This of course depends on the continuous desire of each father to tie up the lands in the family and on the continuous obedience of each eldest son on attaining 21.