CS4FN Advent – Day 25: Merry Christmas! Today’s post is about the ‘wood computer’

Today is the final post in our CS4FN Christmas Computing Advent Calendar – it’s been a lot of fun rummaging in the CS4FN back catalogue, and also finding out about some new things to write about.

Each day we published a blog post about computing with the theme suggested by the picture on the advent calendar’s ‘door’. Our first picture was a woolly jumper so the accompanying post was about the links between knitting and coding, the door with a picture of a ‘pair of mittens’ on led to a post about pair programming and gestural gloves, a patterned bauble to an article about printed circuit boards, and so on. It was fun coming up with ideas and links and we hope it was fun to read too.

We hope you enjoyed the series of posts (scroll to the end to see them all) and that you have a very Merry Christmas. Don’t forget that if you’re awake and reading this at the time it’s published (6.30am Christmas Day) and it’s not cloudy, you may be able to see Father Christmas passing overhead at 6.48am. He’s just behind the International Space Station…

And on to today’s post which is accompanied by a picture of a Christmas Tree, so it’ll be a fairly botanically-themed post. The suggestion for this post came from Prof Ursula Martin of Oxford University, who told us about the ‘wood computer’.

It’s a Christmas tree!


The Wood Computer

by Jo Brodie, QMUL.

Other than asking someone “do you know what this tree is?” as you’re out enjoying a nice walk and coming across an unfamiliar tree, the way of working out what that tree is would usually involve some sort of key, with a set of questions that help you distinguish between the different possibilities. You can see an example of the sorts of features you might want to consider in the Woodland Trust’s page on “How to identify trees“.

Tree silhouettes image by Clker-Free-Vector-Images from Pixabay

Depending on the time of year you might consider its leaves – do they have stalks or not, do they sit opposite from each other on a twig or are they diagonally placed etc. You can work your way through leaf colour, shape, number of lobes on the leaf and also answer questions about the bark and other features of your tree. Eventually you narrow things down to a handful of possibilities.

What happens if the tree is cut up into timber and your job is to check if you’re buying the right wood for your project. If you’re not a botanist the job is a little harder and you’d need to consider things like the pattern of the grain, the hardness, the colour and any scent from the tree’s oils.

Wooden bridge image by Peter H from Pixabay

Historically, one way of working out which piece of timber was in front of you was to use a ‘wood computer’ or wood identification kit. This was prepared (programmed!) from a series of index cards with various wood features printed on all the cards – there might be over 60 different features.

Every card had the same set of features on it and a hole punched next to every feature. You can see an example of a ‘blank’ card below, which has a row of regularly placed holes around the edge. This one happens to be being used as a library card rather than a wood computer (though if we consider what books are made of…).

Image of an edge-notched card (actually being used as a library card though), from Wikipedia.

I bet you can imagine inserting a thin knitting needle into any of those holes and lifting that card up – in fact that’s exactly how you’d use the wood computer. In the tweet below you can see several cards that made up the wood computer.

One card was for one tree or type of wood and the programmer would add notch the hole next to features that particularly defined that type. For example you’d notch ‘has apples’ for the apple tree card but leave it as an intact hole on the pear tree card.  If a particular type of timber had fine grained wood they’d add the notch to the hole next to “fine-grained”. The cards were known, not too surprisingly, as edge-notched cards.

You can see what one looks like here with some notches cut into it. You might have spotted how knitting needles can help you in telling different woods apart.

Holes and notches

Edge-notched card overlaid on black background, with two rows of holes. On the top a hole in the first row is notched, on the right hand side two holes are notched. Image from Wikipedia.


Each card would end up with a slightly different pattern of notched holes, and you’d end up with lots of cards that are slightly different from each other.

Example ‘wood computer’. At the end of your search (to find out which tree your piece of wood came from) you are left with two cards for fine-grained wood. If your sample has a strong scent then it’s likely it’s the tree in the card on the right (though you could arrive at the same conclusion by using the differences in colour too). The card at the top is the blank un-notched card.

How it works

Your wood computer is basically a stack of cards, all lined up and that knitting needle. You pick a feature that your tree or piece of wood has and put your needle through that hole, and lift. All of the cards that don’t have that feature notched will have an un-notched hole and will continue to hang from your knitting needle. All of the cards that contain wood that do have that feature have now been sorted from your pile of cards and are sitting on the table.

You can repeat the process several times to whittle (sorry!) your cards down by choosing a different feature to sort them on.

The advantage of the cards is that they are incredibly low tech, requiring no electricity or phone signal and they’re very easy to use without needing specialist botanical knowledge.

You can see a diagram of one on page 8 of the 20 page PDF “Indian Standard: Key for identification of commercial timbers”, from 1974.

Teachers: we have a classroom sorting activity that uses the same principles as the wood computer. Download our Punched Card Searching PDF from our activity page.


The creation of this post was funded by UKRI, through grant EP/K040251/2 held by Professor Ursula Martin, and forms part of a broader project on the development and impact of computing.



Previous Advent Calendar posts

CS4FN Advent – Day 1 – Woolly jumpers, knitting and coding (1 December 2021)


CS4FN Advent – Day 2 – Pairs: mittens, gloves, pair programming, magic tricks (2 December 2021)


CS4FN Advent – Day 3 – woolly hat: warming versus cooling (3 December 2021)


CS4FN Advent – Day 4 – Ice skate: detecting neutrinos at the South Pole, figure-skating motion capture, Frozen and a puzzle (4 December 2021)


CS4FN Advent – Day 5 – snowman: analog hydraulic computers (aka water computers), digital compression, and a puzzle (5 December 2021)


CS4FN Advent – Day 6 – patterned bauble: tracing patterns in computing – printed circuit boards, spotting links and a puzzle for tourists (6 December 2021)


CS4FN Advent – Day 7 – Computing for the birds: dawn chorus, birds as data carriers and a Google April Fool (plus a puzzle!) (7 December 2021)


CS4FN Advent – Day 8: gifts, and wrapping – Tim Berners-Lee, black boxes and another computing puzzle (8 December 2021)


CS4FN Advent – Day 9: gingerbread man – computing and ‘food’ (cookies, spam!), and a puzzle (9 December 2021)


CS4FN Advent – Day 10: Holly, Ivy and Alexa – chatbots and the useful skill of file management. Plus win at noughts and crosses – (10 December 2021)


CS4FN Advent – Day 11: the proof of the pudding… mathematical proof (11 December 2021)


CS4FN Advent – Day 12: Computer Memory – Molecules and Memristors – (12 December 2021)


CS4FN Advent – Day 13: snowflakes – six-sided symmetry, hexahexaflexagons and finite state machines in computing (13 December 2021)


CS4FN Advent – Day 14 – Why is your internet so slow + a festive kriss-kross puzzle (14 December 2021)


CS4FN Advent – Day 15 – a candle: optical fibre, optical illusions (15 December 2021)


CS4FN Advent – Day 16: candy cane or walking aid: designing for everyone, human computer interaction (16 December 2021)


CS4FN Advent – Day 17: reindeer and pocket switching (17 December 2021)


CS4FN Advent – Day 18: cracker or hacker? Cyber security(18 December 2021)


CS4FN Advent – Day 19: jingle bells or warning bells? Avoiding computer scams (19 December 2021)


CS4FN Advent – Day 20: where’s it @? Gift tags and internet addresses (20 December 2021)


CS4FN Advent – Day 21: wreaths and rope memory – weave your own space age computer (21 December 2021)



CS4FN Advent – Day 22: stars and celestial navigation (22 December 2021)



CS4FN Advent – Day 23: Father Christmas – checking his list, spotting the errors (23 December 2021)


CS4FN Advent – Day 23: Bonus material – see “Santa’s sleigh” flying overhead (23 December 2021) – this was an extra post so that people could get ready to see “Father Christmas” passing overhead on Christmas Day at 6:48am)


CS4FN Advent – Day 24: Santa’s Sleigh – track its progress through the skies (24 December 2021)


CS4FN Advent – Day 25: Merry Christmas! Today’s post is about the ‘wood computer’ (25 December 2021) – this post




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