The TO-237 package is a registered JEDEC transistor outline titled “Header Family, Flat Index,” last revised to Issue B in October 1979. It is not, on JEDEC’s own wording, a miniature plastic package. No vendor drawing, distributor package category or orderable part in TO-237 turns up in a current search. If TO-237 appeared on your BOM, this page tells you what it is and what you are more likely holding.

Key takeaways

• JEDEC’s Master Index registers TO-237 as “Header Family, Flat Index,” Issue B, October 1979. It has not been revised since.
• JEDEC writes “Plastic Small Outline Package” for TO-236, TO-253 and TO-261. It does not use the word for TO-237.
• TO-226 AE carries the identical title, “Header Family, Flat Index.” TO-226 is the registration behind TO-92.
• TO-237 survived the 1987 and 1996 inactivation purges that archived its neighbours TO-239, TO-241, TO-242 and TO-248.
• Most TO-237 search hits are digit collisions: a 2.37 mm lead span in a CAD library name, or a BD237 part number.

What the TO-237 package actually designates

TO-237 is a registered outline in JEDEC Publication No. 95, the catalog of transistor outline drawings maintained by committee JC-11. Its registered title is “Header Family, Flat Index.” The Master Index gives it issue letter B and a date of October 1979, and the 2024 contents index still lists it.

That is the complete public record. JEDEC sells the drawing inside JEP95; the free indexes give the title, the issue letter and the date, and nothing else. There is no published dimension set, no pin assignment and no device example in the open literature.

[IMAGE 1: Excerpt from the JEDEC Master Index showing the TO-236 through TO-240 rows with issue letters and dates | alt: “TO-237 package entry in the JEDEC Publication 95 Master Index showing Issue B October 1979”]

TO-237 in JEDEC’s own index

The registration block around TO-237, taken from the Master Index for JEDEC Publication No. 95:

Outline	JEDEC title	Issue	Date	Status
TO-92	Axial Leaded, Flat Index	A	—	Archived – JEP 95
TO-226 AE	Header Family, Flat Index	A	December 1981	Registered
TO-236	Plastic Small Outline Package (SOT/SOP), 3 Leads	H	January 1999	Registered
TO-237	Header Family, Flat Index	B	October 1979	Registered
TO-238	Flange-Mounted Header, Rectangular	D	March 1982	Registered
TO-243	Header Family, Peripheral Terminal	C	July 1986	Registered
TO-245	Header Family, .100 Pin Circle	—	—	Drawing not available
TO-246	Header Family, .200 Pin Circle	—	—	Drawing not available
TO-253	Plastic Small Outline Package (SOT/SOP), 4 Leads	D	January 1999	Registered

Three things stand out. TO-237 is frozen at Issue B, October 1979 — a 46-year-old drawing, against TO-236 at Issue H in January 1999. TO-245 and TO-246 are registered numbers with no drawing available at all, which tells you how loosely the block is populated. And TO-237 outlived its neighbours: the device-type index lists TO-239, TO-241, TO-242 and TO-248 as archived in the 1996 inactivation, while TO-237 stayed.

Treat the indexes with the same care. JEDEC publishes more than one, and they do not agree on every row — the Master Index puts TO-247 at Issue E, June 2004, while the Main Index puts it at Issue F, August 2022. Where they differ, take the later date and pull the drawing.

Why “miniature plastic outline” does not hold up

JEDEC is specific when a package is plastic. TO-236 is “Plastic Small Outline Package (SOT/SOP), 3 Leads.” TO-253 is the four-lead version of the same wording. TO-261 is “Plastic Small Outline Package SOP/SOT.” TO-273 is “Plastic Flange Mounted Package, 3 Leads.”

TO-237 gets none of that. It is filed under HEADER FAMILY alongside TO-205, TO-206, TO-236, TO-243 and TO-251, with the qualifier “Flat Index” — the flat face used for orientation, the same qualifier JEDEC applies to TO-92 and to TO-226 AE.

The closest reading the evidence supports is that TO-237 belongs to the same header family as TO-226 AE, the registration behind TO-92, and shares its flat-index orientation feature. That is an inference from a matching title, not a specification. Anyone describing TO-237 as a miniature plastic package is going beyond what JEDEC publishes.

What you are probably actually holding

What you saw	What it probably is	How to tell
A library name like SOT95P237X117-3N	TO-236AA, a SOT-23	The code reads pitch, span, height: 0.95 mm pitch, 2.37 mm lead span, 1.17 mm height. The 237 is a dimension
BD237 on a BOM line	A transistor part number, not a package	onsemi’s BD237 datasheet carries the TO-225 mechanical case outline, document 98ASB42049B
TO-237 against a 3-lead surface-mount part	TO-236	JEDEC titles TO-236 “Plastic Small Outline Package (SOT/SOP), 3 Leads”
TO-237 against a 3-lead through-hole plastic part	TO-226 AE, the TO-92 registration	Identical JEDEC title, “Header Family, Flat Index”

[IMAGE 2: A SOT-23 beside a TO-92, annotated with their JEDEC registrations TO-236 and TO-226 AE | alt: “TO-237 package disambiguation showing SOT-23 as TO-236 and TO-92 as TO-226 AE”]

How to resolve a TO-237 line on a BOM

Work in this order. It takes about five minutes.

1. Check whether 237 is a dimension. Search the string for a neighbouring pitch and height. IPC-style library names encode span in hundredths of a millimetre.
2. Check whether 237 is part of the device number. BD237, 2N237 and similar collide with the outline name in text search.
3. Count the leads and the mounting style. Three leads surface mount points to TO-236. Three leads through-hole with a flat face points to TO-226 AE.
4. Ask the supplier for the case outline drawing, not the package name. A drawing number settles it; a catalog string does not.
5. If the line is genuinely a TO-237 package, treat it as legacy. A 1979 drawing with no current products behind it is a sourcing risk, not a specification.

Frequently asked questions

Is TO-237 a real package?

It is a real JEDEC registration. The Master Index for JEDEC Publication No. 95 lists TO-237 as “Header Family, Flat Index,” issue letter B, October 1979, and the 2024 contents index still carries it. What does not exist in the open record is a vendor drawing, a distributor package category or an orderable part in that outline.

What is the difference between TO-236 and TO-237?

JEDEC titles them differently and revises them differently. TO-236 is “Plastic Small Outline Package (SOT/SOP), 3 Leads,” at Issue H dated January 1999 — the registration behind SOT-23. TO-237 is “Header Family, Flat Index,” at Issue B dated October 1979. They are adjacent numbers, not variants of each other.

Is TO-237 the same as TO-92?

Not according to the index, though they are related. TO-92 itself is archived. Its live replacement is TO-226 AE, which JEDEC titles “Header Family, Flat Index” — the identical wording used for TO-237. Same family, same orientation feature, separate registrations. Do not treat one drawing as covering the other.

Why can’t I find any TO-237 parts?

Because the designation is not in current use. A search across manufacturer package libraries, distributor package filters and datasheet aggregators returns no device specified as TO-237. Registration in JEP95 does not imply that anyone builds to the outline, and TO-245 and TO-246 in the same block have no drawing at all.

What to do next

If TO-237 came from a CAD library: check the digits against a lead span before anything else. Most hits resolve to TO-236 the moment you read the name as a dimension code.

If it came from a legacy drawing or a spares list: treat it as an unresolved designation and request the case outline drawing from whoever supplied the part. A 1979 issue letter with no products behind it will not survive a supplier audit.

If you are writing a package field yourself: use the designation the manufacturer’s datasheet prints. Registration numbers that nobody builds to create exactly this problem for the next engineer.

If you need the actual drawing: it sits inside JEP95, which JEDEC sells. The free indexes cited here give you the title, issue and date, and that is enough to decide whether buying it is worth your time. On this evidence, it usually is not.