main anatomy page


Of the several characteristics of grain in wood (as opposed to figure in wood which is more complicated than just grain), such as wavy, straight and irregular, most are not at all indicative of a species. That is, there are no species of wood that always have straight grain or always have wavy grain. BUT ... there is one characteristic, interlocked grain, that is closely related to a few woods, such that if you find what you think is one of those woods but it does not have that characteristic, your identification of the wood is automatically suspect. It isn't necessarily wrong, but depending on the wood, the likelihood of it being wrong if the wood doesn't clearly exhibit interlocked grain can range from fairly high to almost a sure thing.

For example, if I saw a wood identified as verawood and it showed no signs of interlocked grain I would START with the assumption that the ID was wrong because it almost certainly would be. Similarly ipe and sucupira are pretty much sure to have interlocked grain. Billy Webb wood, on the other hand, usually HAS interlocked grain but it is sometimes not nearly as apparent as on it as on or verawood.

Unfortunately most sources I've found for wood information just say whether or not a species has interlocked grain (if they even say that much) but they give no information about how obvious it is and whether or not it is always present in a species or sometimes present or rarely present. I believe that some woods can have interlocked grain or not, depending on the tree, but I do not yet have any formal sources, such as university sites, to back that up.

OK, so what IS interlocked grain? Well, it's like this: some trees, for reasons I have never seen explained (and I have seen some reports saying that there is no known explanation), grow with the tree fibers twisting during growth, in a way that results in a spiral orientation of the fibers up the trunk rather that the normal parallel to the center-line of the tree. On some (perhaps all) of the trees that do this, the spiral changes direction from year to year (or in groups of multiple years), the result is interlocked grain and it is one of the reasons (but usually not the only reason) for such figures as "ribbon stripe" (in quartersawn stock), "mottled", "curly", and others. I think a change in direction every year is the most common but I don't know that as a fact.

Interlocked grain can cause the wood surface to have a high chatoyance. It occurs in many hardwoods, and although it often produces a visually stunning figure, such woods can be difficult to work because the interlocked grain tends to tear out in planing and other operations. Also, such wood is usually exceedingly difficult to split.

NOTE: Correspondent Zach Lichtmann kindly alerted me to some further research on interlocked grain and while I still need to dig in farther, two things from it deserve immediate note. First, yearly switching from one direction of spiral grain to the other is NOT the norm (although it does occur regularly in some species), The cycle of going from as far in one direction as spiral grain is going to go and then gradually changing back to the maximum spiral in the other direction can take many DECADES in some species. This research further noted that spiral and interlocked grain are sometimes associated with situational torque (high winds in an irregular canopy for example) but are also associated with genetics. Now, "genetics" doesn't really explain anything in the sense that it's like saying that some folks have very light skin and some have very dark skin and it's due to their genetics. Well, yes, it IS due to their genetics as the proximate cause but WHY did it arise in the first place? In the case of human skin, it's well understood to have been an evolutionary response to conditions of high sunlight / low sunlight. So what ULTIMATELY causes spiral grain and interlocked grain? We don't know. I hypothesize that it arises out of situational torque and propagates slowly in a species because trees with interlocked grain are likely to be stronger in their ability to withstand storm winds. That is, as I said, purely a hypotheses, with no solid data behind it.

Here's a graphic on the left of what that spiralling looks like internally and then a graphic of what it looks like in a flat cut surface oriented with the grain running up/down as in the spiral graphic. Although interlocked grain is often really obvious on the face grain it almost never is on the end grain (see discussion at the bottom of the page regarding end grain).

internal tree structure
with interlocked grain
face grain of a flat cut piece
with interlocked grain

Interlocked grain is usually most obvious in the face grain of a flat cut piece because there is an obvious shift in the direction of the grain, as seen in the graphic above. Here are a couple of real-world examples, up close.

These are small areas (about 1" x 1/2" shown here at about 4X) with the grain running left to right instead of up-down as in the graphics above but clearly showing the same shift in grain direction across growth rings.

In addition to the exotics shown below, in which the interlocking grain is trivially easy to see, there are several domestic species that commonly have interlocked grain but in which it does not show up so clearly. Chief among these is American elm (and other elms) which frequently has interlocked grain. You may not see the signs clearly the way you will in the samples shown on this page, but if you go to split some for firewood, you won't have any doubt about whether or not it is there. American sycamore also sometimes has interlocked grain, as do American cherry, walnut (included below), and ash.

Here are more examples of what interlocked grain looks like on mostly flat cut surfaces and one quartersawn surface.

flat cut sections of two pieces of Billy Webb wood 3"x2" shown here at about 1.6X

flat cut sections of a piece of ipe and a piece of verawood, both 3" x 2" and shown here at about 1.6X

a section of flat cut sapele 3" x 2" shown here at about 1.6X and a section of quartersawn sapele 8" x 5" shown here at 50% of full size

flat cut sections of sucupira and African mahogany, both 5" x 3" and shown here at 100%


Interlocked grain is rarely noticeable at all on the end grain. One wood where it IS frequently noticeable is verawood. Here are examples:

A pair of 1"inch wide end grain cross sections of verawood shown here at 5X showing how this woods DOES exhibit some obvious interlocked characteristics in the end grain.

For all of the other woods which are shown above with interlocked grain that is obvious on the face grain, here are end grain shots of the same samples, showing that the interlocking just doesn't show up at all on these particular samples of these species. It is also true that it is unlikely to show up on other samples of these same species.

1" wide end grain cross sections of both samples of Billy Webb wood shown here at 5X

1" wide end grain cross sections of African mahogany and ipe shown here at 5X

1" wide end grain cross sections of sucupira shown here at 5X