Writs sent by Henry V to the sheriff of Hampshire (and similarly to Sussex, Dorset and Wiltshire) on June 2, 1417.
Close Rolls, 5 Henry V, m. 15 in dorso:
To the sheriff of Suthampton. Order to cause proclamation to be made in singular at the places within his bailiwick where the king has commanded proclamation of musters to be made, that no man of whatsoever estate, degree or condition shall assume arms or coats (tunicas) of arms called 'cotearmures' unless he possess or ought to possess the same in right of an ancestor or by gift of one having sufficient power, and that on the day of his muster he shall show clearly to persons now or hereafter appointed by the king by whose gift he has the same under pain of not being admitted to sail upon the present expedition under the number of him by whom he is retained, of losing his wages, and of the defacing and breaking of such arms and 'cotearmures' at the time of the muster if displayed or found upon him, except the men who with the king bore arms at the battle of Agincourt (exceptis illis quis nobiscum apud bellum de Agincourt arma portabant); as the king has information that divers men who heretofore in his expeditions have assumed arms and 'cotearmures,' when neither they nor their ancestors used them in times past, and are purposing to wear the same upon this expedition; and although the Almighty dispenses his grace as he will upon rich and poor, nevertheless the king's will is that of his lieges every man shall be entreated as his estate demands. By K.
Like writs to the sheriffs in the following counties:
Wiltshire. Dorset. Sussex.
Prior to that date, many people assumed arms, as the writ itself testified. Formal grant by competent authority did happen, but records of such grants are rare. For those who had not inherited arms, the most common method was probably the bearer simply choosing arms that pleased him. Thus Scrope, Grosvenor and Carminow ended up with the same arms, and wore them for generations before anyone noticed a conflict.
Note that the 1417 writ only applied to those gentlemen that appeared at muster. The formal process of Heralds Visitations didn't start until 1530.
The pre-1417 theory of arms (as expressed by Christine de Pisan) is as follows.
1) If you are born to, or rise through your own virtue to, sufficiently high estate, you may assume whatever arms please you, subject to certain limitations.
a) You mustn't choose arms that are already being legitimately used by someone else in the same country. After the Scrope-Grosvenor case in 1390, England also accepted the principle that the bearer of arms may also prevent others unrelated in blood from bearing arms that are too similar. Nor may you assume arms that legitimately pertain to some town or office. Because there was no comprehensive heraldic database, conflicts might occur by accident. Erasing the conflict required a potentially expensive lawsuit to determine who had priority, so some conflicts were tolerated after discovery.
b) You can freely assume arms legitimately held by someone in another country, as long as no fraud or deception was involved. If an English man at arms who was a pillager and vagabond assumed the arms of some French gentleman so that he could rob and pillage in France or Flanders while hoping the blame would fall on the Frenchman, he would deserve to be hung for his crimes.
Now, the practical question arises, what is sufficiently high estate, and what happens if someone of low estate thinks he is more gentle than he actually is?
There doesn't seem to be formal legal recourse in pre-1417 England for a gentleman to deal with an upstart commoner who assumes non-conflicting arms. However, depending on their relative status, there may be a lot a high status gentleman can do to informally make life difficult for an social interloper.
Consider the poor man-at-arms of humble birth. If he serves in a retinue, his captain has a lot of scope to make him regret presumptuous behavior. Likewise, in peace his landlord or justice of the peace can do so, if he disapproves.
Conversely, if the commoner is wealthy enough to have independent means, or if his gentle companions at arms decide to support and affirm, or at least acquiesce to, his claims because of their shared camaraderie, it becomes a lot easier to assume arms and make it stick.