As well as extension tubes there are a number of things you can do such as 'reversing rings' for the lens itself, 'coupling rings' which fit say a 50mm lens on the front of a telephoto lens. The later leads to considerable vignetting unless the tele has a narrow enough angle of view to look through the glass of the extra lens.
These are tubes which fit between camera and lens, with most importantly, an arrangement to transfer the pressure of the camera to close the diaphragm for the shot. This permits you to focus and set up the shot wide open.
With my Hanimex set using the 50mm lens they give a coverage, long frame dimension, with the focusing range of the lens itself of ....[ Infinity to 0.45m]
#1 86 > 140mm #3 52 > 70mm
#2 60 > 80mm #1+2+3 27 > 30mm
The advent of TTL metering made the use of tubes and bellows so much easier since it automatically copes with the effective size of the diaphragm getting smaller as it is moved forward. With all three one gets closer than 'double extension' or 1:1 At 1:1 for instance f/4 equals f/8.
For example with a digital P&S the vignette makes me wonder what is gained over enlarging part of a shot taken from minimum focusing distance. With a pro-sumer and x8 zoom there wasn't much improvement. If one can go in and take just 3cm across the frame it doesn't seem much point in the exercise.
There is interesting reading on the website of SRBfilm who are specialists in this sort of thing and will make adaptions if they do not already have them for your camera.
www/srbfilm.co.uk
Macro was used and permits one to come in to 10cm from the subject with camera used.