Needlepoint canvas comes in different shapes and sizes.
The type you choose for any particular project will depend upon what you plan to use it for.
Needlepoint canvas is made from cotton and woven into a mesh arrangement.
The mesh has varying degrees of "openness" and this is how the canvas is sized.
For example, a canvas with 18 holes to the inch is called an "18 mesh canvas" and a canvas with 12 holes per inch is called a "12 mesh", or "12 gauge", canvas.
The higher the number, the smaller the holes! Cotton needlepoint canvas can be washed in water but you do need to take care that the threads you have stitched the canvas with are not going to 'run'.
Most threads today are color-fast but a fastness test is advised before washing a stitched canvas.
There are four main types of needlepoint canvas:

• Mono Needlepoint Canvas.
  This typically comes in white or beige, but other colors are available.
  It is a single mesh canvas which means it is woven with a single thread.
  The threads are woven over and under each other in a typical weaving pattern and held together at the junction by sizing (starch).
  Mono needlepoint canvas is a good choice for stitching projects that are going to be used as furnishings, such as chair cushions.
  The mono canvas will "give" at the junctions rather than break under wear and tear conditions.
  This type of canvas comes in a variety of sizes ranging from 7 mesh to 18 mesh for needlepoint.
  
• Interlock Needlepoint Canvas.
  This is also a single thread needlepoint canvas but it is interlocked at the junctions - it looks like one thread goes through the middle of another where they intersect.
  This means there is the potential for breakage, but this is rare.
  It does not unravel as easily as mono canvas, because of these secure junctions, and so it is a good canvas to use for irregular-shaped needlepoint projects.
  It is cheaper than mono canvas too - about half the price.
  Most needlepoint kits are printed onto interlock canvas.
  Interlock canvas is available in most mesh sizes for needlepoint - 10-18 gauge.
  
• Penelope Needlepoint Canvas.
  This is a double thread canvas which makes it a strong option and the best choice for furnishings and projects that will get a lot of wear and tear.
  The other useful thing about penelope canvas is that you can choose to stitch all the holes (this is called petit point) or you can just stitch the big holes (gros point).
  Most stitchers stitch a penelope canvas doing both - they stitch the small holes for the small, detailed areas of the design, and the large holes using a thicker thread for the background.
  This way you can get some beautiful effects from penelope canvas.
  It is only widely available in size 10/20 - this means 10 holes to the inch if you are only counting the big holes (gros point), or 20 holes to the inch if you are counting all the holes (petit point).
  
• Plastic Needlepoint Canvas.
  This is a stiff mesh canvas made out of plastic.
  It is great for teaching children how to stitch and it is what you use if you want to make a three-dimensional project i.
  e.
  something that is going to stand up like an ornament or tissue box.
  Unlike the other needlepoint canvases, plastic canvas is widely available at craft stores.
  

Blank needlepoint canvas is not widely available and you are unlikely to find it at a 'big box' craft store.
This is because needlepoint itself has become something of a niche product.
Blank needlepoint canvas is generally only available from specialty needlepoint retailers or online.
The good thing about this, however, is that these retailers are needlepoint experts and so will be able to help you choose the right product and threads for your project.