For my Pi3 with the 7-inch touch screen, it's a no brainer. Yet the distro that we got with NOOBS didn't come with any. Like everything else with linux, the promise of easy installation was misleading.
First I tried installing "matchbox-keyboard".
However, not initially having access to a network connection, I made the mistake of trying to download the .deb files, for matchbox and two dependencies per this site.
In retrospect, there was no assurance that the .deb files would match my distro. When I ran matchbox from a shell, it spit out piles of Xwindows errors and crashed, leaving the OS inoperable, with no ability to open new windows or even reboot. But I'm not the only moron to encounter this; googling the error message gets tons of hits, none of which have any fixes except 'use xvkbd.'
https://raspberrypi.stackexchange.com/questions/47865/raspbian-jessie-gui-hangs-after-running-matchbox-keyboard
Later, once I was home and could hook up to my own network, I did an update and then an apt-get install for Florence.
After installation, it didn't run.
https://www.google.com/search?q=florence+couldn%27t+connect+to+accessibility+bus&rlz=1CDGOYI_enUS705US706&oq=florence+couldn%27t+connect+to+accessibility+bus
https://unix.stackexchange.com/questions/230238/starting-x-applications-from-the-terminal-and-the-warnings-that-follow
https://bugs.launchpad.net/ubuntu/+source/at-spi2-core/+bug/1193236
However a few forum threads helpfully suggested installing the at-spi2-core package.
forums.pimoroni.com/t/florence-will-not-load/3995/2
https://www.raspberrypi.org/forums/viewtopic.php?f=108&t=123189
Now it worked, and none of the keys were blacked-out which is a problem that I was experiencing before. It is ergonomic and beautiful... except that when I press the ALT key, the entire pi freezes up. The mouse still works but I can't click on anything, and even a physically connected keyboard no longer works.
https://www.google.com/search?site=&source=hp&ei=b7mBWfuME82cmwHm97lI&q=florence+alt+key+crashes+mouse&oq=florence+alt+key+crashes+mouse&gs_l=mobile-gws-hp.3...3236.27726.0.28253.41.40.1.7.7.0.396.3975.24j15j0j1.40.0....0...1.1.64.mobile-gws-hp..0.36.3087.3..0j41j46j0i131k1j0i46k1j0i22i30k1j33i160k1j33i21k1j33i22i29i30k1.c0R5jf7tlbQ
https://www.google.com/search?rlz=1CDGOYI_enUS705US706&hl=en-US&ei=vbuBWdbiMYTSmwHPzLnABw&q=florence+alt+stuck&oq=florence+alt+stuck&gs_l=mobile-gws-serp.3...336137.340736.0.341190.10.9.1.0.0.0.111.801.7j2.9.0....0...1.1.64.mobile-gws-serp..0.9.697...0j41j33i21k1j33i160k1j0i22i30k1.cgo6dkJeLR4
Unfortunately, there aren't a lot results when googling this problem. The one link that I've found so far just has 'use xvkbd' as the workaround.
https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=164844
with no solutions, I decided to uninstall Florence:
https://www.raspberrypi.org/forums/viewtopic.php?f=91&t=9905
