When you're buying ceramic or porcelain tiles, one of the choices you'll have (particularly with bigger tiles) is whether or not to get 'rectified' tiles. This decision will mostly depend on the type of look you're after.
What are 'rectified' tiles?
Rectified tiles are ceramic or porcelain tiles that have been precisely ground and machined to give them near-perfect straight edges and exact dimensions. These tiles provide a very clean, symmetrical look, and allow for extremely fine grout lines of 3mm or less (typically using unsanded grout, which is most appropriate for thin grout lines).
Rectified tiles usually also have a very fine bevel around their top edge too, to help reduce the chance of their sharp edges chipping.
These tiles are also known as 'dimensionally stable tiles' (which is a bigger mouthful and no more descriptive) - and are often referred to as 'sharp edge' tiles because they're quite sharp at the edge.
Because of this sharpness and the likelihood that they'll be chipped, a bit of thought needs to go into how they're used for outward facing edges. Where necessary, bullnose tiles can be used to give softer edges. Conventional non-rectified tiles are called cushion-edged, soft-edged or pillow-edged, and aren't as sharp or prone to chipping on their edges.
For practical reasons, it's unusual to find rectified tiles smaller than about 300mm x 300mm.
How are rectified tiles made?
They're called 'rectified' tiles because that's what the process actually does - it rectifies their sizes. Baking or firing a ceramic tile never gives an exact, predictable result. Even if the tiles go into the kiln looking exactly the same, the change in moisture through the firing process isn't something that can be precisely controlled, and tiles will slightly shrink and warp. When they come out of the kiln, ceramic and porcelain tiles will always vary slightly in terms of their shapes and sizes.
Rectified tiles are normally a bit bigger than they need to be when they're put in the kiln, to allow for them to be cut back to precisely the right size once they've been fired. The 'rectification' process involves either grinding or cutting the tiles with a diamond saw.
Are rectified tiles all exactly the same?
Rectified tiles are practically identical in terms of length and width - the machines used to cut them work to very fine tolerances, and for most people's purposes there won't be a noticeable difference from one tile to the next.
Having said that, the tile's thickness isn't normally taken into account during rectification - only the length and width - so there may be slight variations in terms of thickness. Likewise, ceramic tiles may also be subject to a tiny bit of warping.
Because rectified tiles are usually laid very close together with only a sliver of grout separating them, these small differences may occasionally mean that tiles aren't (or don't seem to be) perfectly flush with each other.
This slight difference where tiles aren't exactly even with one another is called 'lippage'. With thicker grout lines like you'd use with unrectified / soft-edged tiles, this amount of lippage would be barely noticeable, and could be easily masked with a gradual slant.
Your tile supplier should be able to tell you more about what to expect in terms of tolerances for warping, differences in tile thickness and lippage.
What's the appeal of rectified tiles?
Because they're very even and uniform, when you're using rectified tiles only the thinnest of grout lines is necessary (3mm or less). The appeal of a sleek, thin grout line's the main reason people choose these tiles. A 1.5mm grout line in particular can give an almost seamless look with the right coloured grout and tiles, and it's a fantastic effect when it's well done.
Because they're so carefully produced, rectified tiles can be used to create a very clean, even and symmetrical look.
Are rectified tiles expensive?
It's always relative to the quality, type and size of tile that you're after, but rectified tiles usually cost more than non-rectified tiles of a similar standard, because there's a bit more work that goes into producing them. Likewise, depending on the size of the grout line you're after and where you're laying them, you may also need to have them laid on a special bedding or substrate to ensure that they're suitably flush.
1.5mm vs. 3mm grout lines
Some tile installers are uncomfortable doing extremely thin grout lines. If you're after the 'seamless' look, you'll need to ensure that you've specified a 1.5mm grout line in the contract rather than leaving it up to the installer, who may otherwise install a 3mm line. Talk to your installer before anything's agreed upon, and make sure everyone's clear about what's required.