Early picture postcards of the Largo area, such as the one above, featured what is known as an "undivided back". This was where the entire back of the postcard was dedicated to the recipient's address. In fact there was a clear printed instruction "the address only to be written on this side". Should the sender wish to add a note, this would have to be squeezed into whatever space might be available on the front. The picture dominated the front of the postcard but sometimes a border or dedicated area was left empty for a brief message to be added.
The example above was posted in July 1901 and generously left half of the front available for a message. Note that the sender was staying at the Belmont Hotel. The example below was sent in October 1900 and was a postcard published for Malcolm's Stationery Salon, Leven. It was also generous with the space left for a message - unlike the one further below where only a lower margin was available and the sender chose not to include a message at all. In the case where no message was added it was presumably sufficient to show the recipient where the sender had visited and provide an idea of how the place looked.
In the UK, the undivided back was the standard for postcards until around 1902, when the "divided back" era took off. This format allowed for a message to be written alongside the address on the rear - the back being divided into two sections, the left section being used for the message and the right side for the address. The piece below from the 24 November 1902 Evening Despatch explained the change at the time under the headline "More room to write".
When divided back postcards became the norm, the old stock of undivided back postcards could be modified. Such an alteration can be seen in the example below. The sender has carefully added a pen-drawn line down the reverse side, before writing a message on the left hand side. In this case, the space on the front below the image was also used. The images on picture postcards soon filled the entire space on the front side and messages became confined to reverse side. The only exception perhaps being the popular 'X' added to mark the spot where the sender was residing.