It was hard to think of one theme, because there are so many beautiful stamps and so many themes to choose.
First I thought of musical instruments, always a favourite theme, and Hawwa inspired me by this blog post (it's this year's theme for Europe!) and by sending beautiful Spanish stamps showing string instruments, like these ones.
But then I thought that 'Musical instruments' might have been, or will be one day, a theme for Viridian's Sunday Stamps already. As have been/will be themes like 'Postage stamps on stamps', 'Mailboxes' and other popular themes for snail mail lovers like us.
So it would be good to choose something else. Finally I found it:
Telephones!
These three stamps have been issued in 1994 'for the elderly':

The present main Dutch telephone company ('KPN') used to be part of one national government-owned enterprise, which also included the Mail company: PTT (Posterijen, Telegrafie en Telefonie: mail, telegraph and telephone).
In 1989 the PTT became independent of the government, and the name changed into KPN (Koninklijke PTT Nederland: Royal PTT Netherlands), including PTT Post and PTT Telecom as main companies.
The state was the only owner until KPN went to the stock market in 1994. In 1998 there was a dissociation: the mail went on in a company named TPG, later TNT, now PostNL, and the telephone branch as KPN.
These stamps have been issued for the aforementioned milestones in 1989 and in 1994:


Here you can see a complete Telephone:

And how times have been changing! Nowadays most telephones have lost their 'line'.
This stamp, issued as part of a 10-pieces stamp sheet for the 100th anniversary of the Dutch Open Air Museum, is titled 'youngsters with a smartphone':
